Number one would be to regain a position from which you can effectively fight from. If you're face down and someone is sitting on you, it isn't hard to roll onto your back. You're going to have to obtain a position where you can fight from your back (the "guard" position).
Often times when you do this, your opponent will roll with you (you can't wait until you are still to begin this roll or it's going to be more difficult -- but not impossible -- to do.
You will then have to escape from his back mount if the opponent is skilled (much easier to do if you are face up). Once you have a stable position to fight from, launch your attack and work to get back up to your feet.
It's simple to write about, more difficult to execute, especially if your opponent is skilled.
If you REALLY want to learn how to do this, join a Brazilian jiu-jitsu club or a wrestling club and actually DO it. The experience is worth more than 10 thousand words on the subject.