wipe with a prep solvent...it will remove wax / other debris...prep is key...sand for mechanical adhesion...150-300grit for primer...then 300-400grit before the first few coats of color...once the color coats look good...sand with 600-800grit before the last few coats of color...wipe with a prep solvent between coats...tack rag after solvent dries. it is a repetive process...when using spray cans don't try to get gloss coats right off the bat...put down a few medium dry coats...then aim for a gloss coat (it will prevent runs + it will cure faster)...if you see any sand scratches or feel them address them before putting clear on because the clear will maginify the scratches and the result is a waste of your time...once the color coats look cool...smooth, no scratches, and possible use a bright light to ensure that you have coverage...wait a day and put on a few medium coats of clear...again you can sand the clear with 800 grit to knock down some imperfections (be very careful not to break through to the color) and then add more clear
these steps, with practice / spray technique will produce a nice paint job...the majority of the time is prep + keeping the enviro clean...use a fan / mask to keep your lungs clean...at some point painting is the easy part...prep is the hard part / time consuming.
expensive products are more durable + have a higher pigment concentration / flashy effects...if you are doing a soild color you can get a 1 stage color / clear coat...reduces dry times, but might not match your part color.
find a body shop supply for sand paper etc and ask them for tips as well...good luck