Here is the easiest, least expensive way to paint your bike. It can get a lot more complicated, Complicated is better, but this might do unless you're wanting a prize winning looker.
Wash the bike good with soap and water. Towel down and let dry.
Take off the wheels and anything else that'll come off without too much trouble. Tape up everything that's left that you don't want paint on.
Wipe down good with paint thinner to remove all residual grease/oil. Using fine sand paper, lightly sand so that new paint will stick on well.
Do some practice sprays before spraying the bicycle. Don't paint unless there is no wind, the humidity is less than 60%, and the temperature is above 60. Hang the frame on a string. Much easier to spray.
Buy the paint at an auto supply store where they're more likely to have the best paint.
Seal with a layer or two of clear coat.
