Clean, brush, sand, clean, primer, sand, clean, paint.
you can do it all in one day if you have time for the primer to dry enough to sand, but it may be better to wait a day before sanding and doing the final painting.
if you have rust you definitely need to put down some primer before you paint.
don't forget putting a clear coat on is a really good option if you want extra protection or a nice gloss finish.
Prep is everything when painting. If you prepare the surface properly your paint should come out fine, if you dont it will look like crap and wont last.