What do I have to do to prepare my garage floor for paint?
I purchased a garage floor epoxy kit It contained very detailed instructions on how to prep the floor. It also had muriatic acid in it to etch the floor before painting on the epoxy.  

One tip I would give is to have someone help you put it on.  You only have a limited time to work with the epoxy once the two parts are mixed. If you have one person cutting in and one person rolling on the main part, it is much easier to get it done in the setup time of the epoxy.