i got the primer, now what???

ok, currently my trucks primered cheaply (rattle can). thats why it looks like shit. but now i got a paint gun for xmas. today i bought a gallon of primer and i wanna shoot it on my truck. what steps do i take? what size sand paper, coats and best way to clean my gun after. my trucks bodies straight, with little bondo at a place where the previous owner had repaired. also what PSI do i need to run it. thanks for any help. wanna try to do the bed tomorrow.