How I prep a concrete garage floor for painting?
I have found an acid etch is about the best and easiest methods. Most masons use muriatic acid at 15-30% with water you can go stronger but this is normally strong enough to etch the surface looking at your photo 15% may be plenty. I use a 2 part epoxy made for garages , don’t go cheap or you may find where you park your vehicle the paint peeling where you stop when the tires are warm. With plain concrete paint I have seen it peel up along the tire path not just where the vehicle stops. Since you are in a garage there should be plenty of ventilation and ringing is easy with a squeegee (that’s what I use) a floor squeegee. I would caution a really nice coating looks super but I spilled some antifreeze on mine one day and it was so slick I could not stand up so now I add a traction powder, you really can’t see it but it is not so slippery.

Remember aaa always add acid to water to prevent an exothermic reaction and possibly having the acid splatter when it boils. 
muriatic is used in swimming pools to adjust ph all pool stores have it as do most home improvement stores for cleaning and etching concrete.