The manifestation of the gift of tongues happens in a few ways.

1) It can happen in such a manner that the speaker is literally speaking a different, and unknown language. This is typically seen in evangelistic endeavors in the Bible and down history.

2) For prophetic utterance. Jesus will pray in tongues through a believer and then give the interpretation of that utterance (either through the one who originally spoke it or through someone else).

3) For edification, or to soothe one's soul and build one up strong for difficult situations.

What you say doesn't matter. You can even repeat the same sounds over and over.

You can ignore all those who wish to put fear in those who would do this, you can trust that God isn't going to let you do something that is bad for your relationship with Him.

If you do it for extended periods, you will find how soothing it is and how much peace will bubble forth in your hearts.