Power Wash it thoroughly. Rub your hand on it. If it's chalky, then your first coat can be primer or paint, but MUST have Emulsabond added to it. Do Not skip that step or you will be in trouble.
If you wash it and its still chalky you did a bad job washing. You must wash until there is NO chalk left. Emulsabond is an additive that will void the manufactures warranty of the material.
A good rule of thumb is wile you are washing - look at the water running down the siding. if its chalky then its still dirty. wash until the runoff is clear. This may take some agitation with a stiff bristled brush and some cleaner to make the job go faster. Prep is very important when coating any substrate.
you do not ever have to use a primer on aluminum siding (unless you are painting safety red - then a gray primer will make it cover better)
apply two (2) coats of high end exterior acrylic satin with an airless sprayer. Do not brush the material - it will look like crap.
so, unless you own a 3200 psi (or higher) pressure washer, an airless sprayer, ladders and masking tools you should probably hire someone.
I have painted over 300 aluminum sided houses in the Cleveland area. the only time I have had material failure is when i painted a previously coated house that was not prepped properly. That is why most painting contractors will not warranty a previously coated substrate.