How do i prep for a painting and everything?

i know how to paint, ive done it before, but as a very young artist with no classes ever taken, i want to know how to do it professionally/correctly. first of all, before you paint should you paint the whole canvas black so you will have completely covered it in paint and theres no thin areas once your done? and if so, i have done this but the black paint wouldnt lay on evenly no matter what ive done. secondly, is there some sealer or something you should put on it after you are finished? and what about the coats, how many should there be? thanks!