Get a scotch bright pad maybe some more abrasive sandpaper (wet-dry if you can) wet-dry paper is good because you get the paper wet and that allows the water to take away the sanded off particles so it doest clog up the paper making it useless masking tape some sort of bug and tar remover and your paint of choice. And maybe some high build prime if you have some curb rash
1. Sand your rims with the scotch bright pad no need to take the paint completely off
2. If you have curb rash get some more abrasive sand paper (like 120 grit) knock off the rough edges tape off the tire and or what ever you don't want paint on it will probably take a few coats but the paint will fill in the rash and then get like 220 grit sand paper and light sand the prime to get it smooth
3 wash your rims
4. Make sure the rim as no water anywhere
5 paint the color of your choice do enough coats till your happy. Then do one more
6 set in sun to dry and or leave in a dry dust free place over night to dry
7. Un mask rims and bam you have new painted rims!