Makeup Primer? How do i use it?

My question is about makeup primer. They sell eyeshadow primer, lip primer, face primer and it gets confusing to me...So is there a primer that works for your whole face? Or is it neccesary to "prime" every area with its individual primer? I've never used it before and i need to because its soo hot and my makeup just melts off! Someone told me using a primer would help, Also is there a good primer for oily skin? Oh and im talking "drugstore" primers only...