Never use paint for a garage floor. It almost always peel off in less than a yr. I recommend a floor stain (water based) or a epoxy kit made for that purpose. You got to strip off all that paint off (solvent or a machine) than power wash it very good with maybe a degreaser for some spots (oil leaks , tire marks etc..) and if the floor is very smooth than you need to acid etch it for first if your going to use a epoxy system. But remember after you clean it let it dry for a few days (2-4). On the floor stain it's more easy to apply but it doesn't clean well. Go with a epoxy kit. (not from lowes or HD)...hopes this helps..

....also stay off the floor after you apply the epoxy for at least 3-5 days (curing)