Move everything out of the garage. You will come across things you do not need anymore and have been lying around for ages. It is the perfect time to get rid of these. Move things you need to the backyard or another room.
Sweep the floor with a broom or leaf blower. Take things off the wall.Wash the room with a power washer or simple hose. Let it dry before proceeding. If there are any holes or cracks in the walls, fill them up with a sealant or joint compound. Smooth out the surface with a sand paper.
Use a roller to apply a layer of primer over the garage walls and floor. Do the walls first of course and than the floor while you are waiting for the walls to dry. This layer of primer gives a lasting touch to your paint. you can use a brush for surfaces that the roller cannot reach.
Get exterior paints which are more heavier than interior ones. Let the primer dry thoroughly. Apply a light coat of the paint with a roller or brush. Let it dry and than proceed to another coat. You will have to apply multiple coats because most garages are made up of cinder blocks which drink up paint.
After you are done painting the walls, use the waiting time to paint the floor. Fill up any cracks using joint compound. Allow the sealant to dry. Sand the surface smooth. Apply a coat of primer and let it dry. Paints for concrete floor are available. You can select any of these to give your garage a complete make over. For a floor, usually a roller with long handle is better.