As painting any surface I would first prep the area and the surface well before I applied a paint or a primer.I'd suggest you also wash the surface of the door with a light scrub and soap. Prepping very important when it comes to getting good results will painting. Your first coat should be oil based primer paint that is a bit thinned down. (The recommended mix is one pint of paint thinner for every one gallon of oil-based primer) Apply paint only after the primer is thoroughly dry. Choose a nice warm and sunny day for this job.
Ask at your local Sherwin-Williams store for the best paint for an aluminum garage door. Also, Yamini is right on about proper prep. You might even power-wash it because exposed exterior doors get so dirty, and you want it to be really clean before you start painting.
If the paint is chipping you may need to do some sanding too...as with any paint project good prep results in good results.
Michael, I'm assuming it's an older door and you just want to change the color. If this is the case, take a scrub brush with soap and water to clean it. After rinsing and drying, then you are ready to apply two coats of top-line acrylic metal paint like Sher-Will 'Direct to Metal'.
Metal is tricky to paint in the cold, so you may want to wait till it warms up. I am also assuming that their is no current peeling and the existing finish is not glossy.
You also want to tape off the bottom gasket so you don't get paint on it. As for the inside of the panel edges, I would advise not to paint them; with acrylic paint on aluminum, they tend to stick even when dry and will pop when you open the door.
Michael, Peace is right on about what was said. Just wanted to add that you should paint the door when it is not in the direct sun or hot to the touch (though a warmer surface is preferred to a cold one). The DTM painPeace talks about is pretty expensive as it is more of a specialty paint, expect to pay $50+ per gallon. It is great stuff though. The cost is roughly the same, or perhaps a bit cheaper, than buying seperate gallons of primer and finish paint added together.
i asked painters. latex works just pressure wash door first. paint it when warm. not hot!!