Your first step is to clean the patio off. Using a pressure washer will be the easiest way to do this. It will remove the dirt and grime and any loose paint chips. If you want a smooth surface, you may need to sand the surface in a few spots where the old paint has chipped. Use a 150-grit sandpaper to smooth out the surface.
Using an Orbital Sander for larger surfaces is a good idea. Use a damp cloth or mop to remove any debris from the sanding.
If the surface is in poor condition, you may want to use a concrete
primer such as Rust-Oleum or BEHR prior to painting your concrete.
Glidden has a great Gripper paint for surfaces to prevent slipping on damp surfaces.
Christine from our Atlanta Office has a great previous post on Painting a concrete floor:
Please let us know if we can be of further assistance.