How can you talk in tongues?

You can't just "talk in tongues" as such an act is a gift from God. Most of the Pentecost Faith believe that you must speak in tongues in order to be saved. This occurs when you repent of your sins and give yourself entirely over to the Holy Ghost and God speaks through you.

Well, where does tongues come from? It comes from Heaven! Tongues is the heavenly language. It is what is spoken in heaven; the only difference is that the people in heaven understand what they are saying. Here on earth Paul says, "For anyone who speaks in tongues does not speak to men but to God. Indeed, no one understand him; he utters mysteries with his spirit"

Also notable is:

When the day of Pentecost came, they were all together in one place. Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. They saw what seemed to be tongues of fire that separated and came to rest on each of them. All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them. (Acts 2:1-4).

I must add, there may be many out there that doesn't believe in such things as "speaking in tongues", but it is an honor and a Gift from God. I have been blessed with this and I have to stay that anyone who believes, but has not experienced it, will have a most wonderful and divine experience when they do.