I bought a kit from home depot to do it - thought it'd be quick, boy was i wrong.
As MrDudeMan said , you have to "etch" bare concrete, so that the primer/paint will stick better, and before you do that you ahve to wash (And completely dry) the floor.
Steps/time for my 2.5 car garage:
1. Empty garage, sweep floor (1.5 hours)
2. Pressure wash (oil, etc) (1.0 hours)
3. Sweep (30min)
4. Etch (2 hours)
5. Wash again (1 hour)
4. Sweep again (30 min)
5. Paint (1.5 hours)
It's quite a job, maybe i went overboard but when i was done it looked really really good. Make sure you wait 24-38 hours, as MrDudeMan said.