If you like myself have been brought up in a pentecostal church, then you might think you know the answer to this, but keep on reading there may just be something to learn here. Let me start by saying that there are 3 reasons for speaking in tongues, but that is getting ahead of myself, let me answer the first part of this question first. Who is the Holy Spirit.

The Holy Spirit is the third part of the trinity of God, we have God the Father, God the Son and the Holy Spirit. The Holy Spirit is the power over the earth that is stopping the devil from doing whatever he pleases. The Holy Spirit was present when God created the earth and universe, In the first chapter in Genesis we find the Holy Spirit is hovering over the waters waiting for God to Speak. The Holy Spirit is the power house of God, given to the church by Jesus at Pentecost after Jesus had ascended into Heaven. Jesus called the Holy Spirit, the helper.

It is the Holy Spirit that gives you the gift of speaking in tongues. There are many other gifts the Holy Spirit gives, but we are only talking about tongues here.

I have already mentioned that there are 3 types or reasons for tongues, so I will deal with and explain what each is and they are in no particular order.

Personal Youngs Literal Translation: 1 Corinthians 14:1-4
1: Pursue the love, and seek earnestly the spiritual things, and rather that ye may prophecy,
2: for he who is speaking in an unknown tongue — to men he doth not speak, but to God, for no one doth hearken, and in spirit he doth speak secrets;
3: and he who is prophesying to men doth speak edification, and exhortation, and comfort;
4: he who is speaking in an unknown tongue, himself doth edify, and he who is prophesying, an assembly doth edify;
This is for your personal edification, God is the only one who understands what is being said, and to make it clear this is your voice speaking but the Holy Spirit giving you the words. When you know you have to pray for something or someone but you do not know the words or how to phrase it, when you wish to worship God but do not know the words to use because your own words are not good enough, then go into tongues because the Holy Spirit knows exactly how to pray for the situation.

Proof Youngs Literal Translation: Acts 2: 1-6
1: And in the day of the Pentecost being fulfilled, they were all with one accord at the same place,
2: and there came suddenly out of the heaven a sound as of a bearing violent breath, and it filled all the house where they were sitting,
3: and there appeared to them divided tongues, as it were of fire; it sat also upon each one of them,
4: and they were all filled with the Holy Spirit, and began to speak with other tongues, according as the Spirit was giving them to declare.
5: And there were dwelling in Jerusalem Jews, devout men from every nation of those under the heaven,
6: and the rumour of this having come, the multitude came together, and was confounded, because they were each one hearing them speaking in his proper dialect,
When the disciples were filled with the Holy Spirit and spoke in other languages, every person in Jerusalem that day heard in their own language, know we do not know and are not told, if this was because the Holy Spirit spoke in their language through the disciples but it is common that the listener heard what was being said supernaturally in their own language Remember that most of the people in Jerusalem were devout Jews who did not believe in the Messiah at the time, and they were there in Jerusalem because it was the Feast of Pentecost which is one of the mandatory pilgrimage feasts. So we can then say that this use of the gift is proof to the unbeliever.

Prophetic Youngs Literal Translation: 1 Corinthians 14: 26-28 and 1 Corinthians 12:7
26: What then is it, brethren? whenever ye may come together, each of you hath a psalm, hath a teaching, hath a tongue, hath a revelation, hath an interpretation? let all things be for building up;
27: if an unknown tongue any one do speak, by two, or at the most, by three, and in turn, and let one interpret;
28: and if there may be no interpreter, let him be silent in an assembly, and to himself let him speak, and to God.
7: And to each hath been given the manifestation of the Spirit for profit;
This one is to edify the church and must have an interpreter in place to interpret the message spoken out. The speaker will give out a message in tongues, and it may easily be that the speaker has no idea what they are saying or to who. The interpreter will be given the interpretation of that message. For the majority of the time the message will be for the congregation or in some circumstance for an individual within that congregation.

There are a lot of churches today who say that speaking in tongues is not for this time or they do not believe it is for them. Some of them have never experienced, others have no idea how to receive the gift even though the Bible clearly tells them how to.
Youngs Literal Translation: Acts 2: 17: And it shall be in the last days, saith God, I will pour out of My Spirit upon all flesh, and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams;
18: and also upon My men-servants, and upon My maid-servants, in those days, I will pour out of My Spirit, and they shall prophesy;
The gift is freely given, you only have to ask

Notice there is no this is male only, verse 18 clearly states, men servants and maid servants, there is no mention that this is reserved for only the top pastors and evangelists. Be very clear this is a gift for everyone who accepts the salvation of Jesus. Let me clear up another little wrong belief a lot of Christians have. Where does this happen, where are the disciples filled with the Holy Spirit? Did you say the Upper Room? Ooopps, yes that is what is taught in a lot of churches, but let us go back to the Hebrew and the traditions of 2000 years ago. Look up a little and in Acts 2 verse 2 above you will see my favourite three letter word "the", this is why most christians get the wrong place it says "and it filled all the house where they were sitting" they assume that "the house" is the house with the upper room. They forget women cannot be in the same room along with the men especially when they are not family or married to one of the men. Then who has a room that will house at least 3000 people? So let me say the "the" we have here is another one of those definitive "the" we are talking about The House, the one and only house, The House of God or The Temple, the desciples are sitting, standing in the Temple courtyard waiting, as soon as they are filled with the Holy Spirit then thousands, will have heard them speaking in tongues. Think about it, the chapter says about 3000 were added to their number that day, now ask yourself how many people do you have to preach to, to get 3000 plus to accept the Messiah as Lord, to believe and repent? The only building in Jerusalem back then which could hold that number of people is the Temple courtyard. The reason so many are there is because this day is one of the feasts of the Lord, the feast of Pentecost, it is a pilgrimage feast so all Jews are requiered to be their, hence the reason so many different languages are required. verses 9 through to 11 of Acts chapter 2 tells us the different countries and what was said:
9: Parthians, and Medes, and Elamites, and those dwelling in Mesopotamia, in Judea also, and Cappadocia, Pontus, and Asia,
10: Phrygia also, and Pamphylia, Egypt, and the parts of Libya, that are along Cyrene, and the strangers of Rome, both Jews and proselytes,
11: Cretes and Arabians, we did hear them speaking in our tongues the great things of God.'
Each person heard the same message in their own language, this was not a Prophetic message nor was it a Personal to the Apostle message, this was a Proof message to the individual who spoke that particular language, no translation was required and there was no personal edification for the apostles.

The desciples had been told to stay in Jerusalem, they knew the gift was coming, they may not have fully understood what it meant, but they knew they would be emboldened with Holy Spirit power.
Acts 1:8 - But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.

Peter the main spokesman in Acts chapter 2 makes it clear what is required of the people who hear the message
Acts 2:38 - Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. We already know that there are hundreds of mikvas at the base of the temple mount, many have already been excavated. So those 3000 would have repented to God, run down the steps and been baptised in the Mikva and as they came up out of the water filled with the Holy Spirit and started talking in tongues.

There is also this report from Paul whilst he is in Ephesus
Acts 19:6 - And when Paul had laid his hands upon them, the Holy Ghost came on them; and they spake with tongues, and prophesied.
So a holy Spirit filled person can also lay hands on fellow believers to recieve the free gift. Notice that the people did not just speak in tongues, they also prophesied the two go hand in hand. Have you repented of your sins and made Jesus the Lord of your life and want to speak in tongues?, then quietly ask Jesus to fill you with the Holy Spirit and start letting your vocal cords make the sounds that rise up from your inner most spirit. It does not have to make sense to you, you will not understand what you are saying most of the time, it is not a language you have to learn, this is simply your inner utterances being heard by God. The one problem most christians have that stops them from speaking in tongues is, they are trying to understand and learn the language first. Sometimes I have just told a person to start saying "hubba hubba hubba aba key no say wa", repeat, repeat and again repeat, then when the brain switches off the spirit takes over and you are tongue talking. You see your voice cant say something unless you open your mouth and your vocal chords make a sound, God understands what you are saying perfectly, you will not. Some people just need a kick start to start talking, for others it happens totally naturally but there is nothing wrong with starting as I mention above because some people allow their brain to get in the way and believe that they need to learn the language first.
Romans 8: 23: And not only so, but also we ourselves, having the first-fruit of the Spirit, we also ourselves in ourselves do groan, adoption expecting ...
26: And, in like manner also, the Spirit doth help our weaknesses; for, what we may pray for, as it behoveth us, we have not known, but the Spirit himself doth make intercession for us with groanings unutterable,
So it may just be a simple groan that rises up inside us, a groan or moan that has no meaning to us, but it means a heck of a lot to God.

Exercise your spirit, grow your spirit. A scientific study reported in 2006, Nov 07 in the New York Times from a study at the University of Pennsylvania shows that praying in tongues has certain benefits to you, it changes the Brain, boosts your immune system, aides in solving problems, making decisions and makes you emotionally stable and that is from a secular newspaper. So when you pray in tongues it is perfect faith, perfect prayers, understanding only by God, gives you access to supernatural gifts, first thing that happens is trhat your getting emmotional healing, then and only then can you move onto all the other supernatural gifts of the Holy Spirit.