I have mine pierced, had it done over a year ago. It doesn't hurt at all! They seat you in the chair, mark your tongue (sometimes they have you lift your tongue up to make sure there aren't any major veins in the way) and then clamp it, and take the needle out and just stick it through, they keep it clamped and take the needle out and insert a large barbell to allow for swelling, then they must screw the ball on, which could take a while depending on how large the piercers hands are. It could start swelling that hour, or the next day. You usually can not talk very well for about a week, and can't eat most food, spoons and forks are hard to get around it seems, and you may wonder how you will ever be able to eat again! Suck on ice chips to help with the swelling and take some ibuprofin or motrin, also try not to play with it much or it will get irritated. Tongue piercings are healed in about 2 weeks.
I also have my belly button pierced, it hurt much more then my tongue. Beware with both these piercings, if pierced to shallow, your body could reject it, my belly button which I have had for 2 years, is being rejected still, which means the barbell is getting closer and closer to the top layer of skin. If this happens to much soon it will fall right out. Tongue rings need to be pretty far back as to not be seen at a job and so they don't get rejected. You can also get what is called a retainer, they are usually plastic and have a flat head, so they can not be seen.
Basically just make sure you find a reputable piercer, no matter how much it costs.
I am sorry for writing so much, if you have any more questions feel free to message me!