This is a relational gift, so it is somewhat different for everyone. But here I want to share some observations I have made from my own experience.
1. Human languages - While praying in tongues, you might suddenly realize that you are speaking another human language. It's a very fun experience.
2. Manifestations - While the act of speaking in tongues is itself a manifestation of the Spirit, you might find yourself shaking, crying or laughing in the Spirit as you pray. It can be a petty wild time!
3. Spiritual downloads - The act of praying in tongues is an opening of yourself to be possessed by Holy Spirit, so do not be surprised if He sends more to you than just words to speak. You can receive prophecy, words of knowledge or wisdom, you might recall scripture passages or just see things in life more clearly than before. When you are that deeply connected to Holy Spirit, there is no telling what He is going to put in you!
4. Comfort - Do not be legalistic or religious about how you speak in tongues. Do whatever comes most naturally and comfortable to you as a person. It might seem more spiritual or humble to lay flat on your belly with your nose to the floor, but if you are uncomfortable you will be distracted from Holy Spirit. Just remember that He is not impressed by religious stuff.
5. You can do it anywhere - I speak in tongues at home, at work, at church, in the car, shopping, in the shower... everywhere! Once you have experienced this gift, it is hard to turn it off. But remember that the point of the gifts is edification (1 Cor 12.7), so please do not speak tongues out loud if there are people around who will hear and be offended or weirded-out. The last thing you want is to drive people away from Jesus because of your practicing a spiritual gift in an inappropriate way. I have found that it is easy to pray in tongues under my breath or even silently while around the "uninformed or unbelievers" (1 Cor 14.23). My only tell is if I get too into it and start laughing or shaking! That's a little awkward...
6. There are no rules - Let yourself be swept up by Holy Spirit! There are no rules when it comes to communion with God. There is no right or wrong way to do it when He is calling the shots. Just let Him lead and you enjoy the ride!