is it ok to use songs in your movies? Are there does and do nots of using people songs? Like lets say i want to use 99 balloons? Could i do that?

It's illegal to use anything which you do not have a written license to use. Of course, in student films, people hardly ever get a license and no one seems to get in trouble. If you're entering it in festivals, or making money off of it, you're going to need a legal document.