I suggest using Chemtech 1L Concrete And Driveway Cleaner. It is biodegradable and non-toxic and does not affect plastic, rubber, glass, and most metals. Please note that this is a concentrate and must be diluted in water. Use a stiff bristle brush broom to remove dirt and debris while using Chemtech.
After brushing the garage, I recommend using a pressure washer to remove deep-seated dirt. This will wash out all the dirt and contaminants on your floor. If you decide to paint it, I suggest using White Knight Ultra Pave Garage Floor Primer and White Knight 4L Medium Grey Ultra Pave Quick Dry Paint. for a quick refresh of the garage floor.
Preparation is key to laying a fantastic looking epoxy coating on your garage floor. Do you know if a sealer has been applied to the concrete already? If not, you can do a simple water droplet test. If the droplet is readily absorbed, then you are good to coat the floor. However, if it just sits on the surface, you'll need to acid etch it before proceeding.
I’ve just finished painting my garage floor with two part epoxy and I'm so happy with the results. If you can afford it I’d definitely recommend epoxy over normal paint. Whatever direction you take you’re going to need to prepare the surface first by degreasing and then probably an acid clean so the paint has a nice clean and oil free surface to stick to. There are plenty of how-tos online.
Get yourself some degreaser, a stiff broom and some elbow grease. There’s a water pooling test you can do to make sure there is no more grease on the floor. Once it’s all degreased then acid wash using a concrete acid wash, which is basically a weak hydrochloric acid. Follow the instructions on the container. Mine was 10:1 mix with water in a plastic watering can and was poured directly onto the concrete from the watering can and brushed with the stiff broom. The concrete will bubble slightly then stop. After all done wash off with the hose giving a good scrub in the process. You will need to make sure concrete is dry before painting - Seaton
A clean up of the floor should be all you need to do. Once the epoxy and the flakes are supplied it is recommended to wait about 24 hours before putting your car back in the garage, and from memory you need your garage open for quite some time after applying it to let it all air out. - maknilsin