I hate to say it...but please do a search...or at least look around.
This question (and what airbrush should i buy) get asked VERY frequently. People take time to write responses, so it would be nice if people would take the time to look for and read them.
I use hardware store primer, either black, white or grey. The krylon stuff that is about $3 a can. I use the flat as I like how it covers - I don't care for the somewhat satin finish of the
gw primers (and their insane prices).