Lots of topics on here about painting aluminum siding.Aluminum Siding Questions
Pressure wash to get the dirt and chalk off the siding. You may have to use a scrub brush and some Krud Kutter cleaner to get any stubborn mildew off. If you have hardened pine sap or some moderate flaking, you could use an orbital sander to remove some of that being careful not to go to hard with the sander because you will end up with swirl marks.
Once it's thoroughly cleaned and DRY........two coats of your favorite exterior paint does the trick. You can spray, brush, roll, whatever method is comfortable for you. I like SW's SuperPaint Satin for aluminum and I like to brush and roll it on. Have done it this way for well over 30 years with great results.......in fact, I've never had to go back to a job because it peeled off the aluminum. I do go back because the paint faded or the house needs a fresh coat. Most aluminum sided house paint jobs last 8 - 10 years, some even longer if there not in direct sunlight most of the day.