I'm not very good myself, but I know a common mistake is to buy a shitload of cans at the beginning.
You don't know if you're really gonna like this, so just buy like 3 colors at first.
A can of black, a can of white and one other color of choice. Brand doesn't really matter, as long as it isn't dirt dirt cheap.
Try to experiment with that. Style and form usually go over color. Crazy color combo's can come later.
As for the caps (nozzles), just get the medium ones, or skinny if you want.

 

I personally say start from the bottom! Dirt garbage cans with stock tips (nozzles) .

 

​don't.

 

I'm by no means an expert. And judging by your post, you're more of an artist than I'll ever be. But what I can say is there is a difference between Graffiti and Street Art. Do a little bit of homework on both crafts and go from there. Do a little digging on the Internet and there's no reason you won't be inspired by both.
As far as graffiti itself goes, I can only say that I personally stick to secluded spots bc I'm not that good with paint, and I haven't used anything other than black or white yet. It's enough to see how your hand development is coming along each time you go out.
And I haven't used REALLY cheap paint. I've used $3 cans in the past but Rusto and Krylon aren't much more expensive but the quality is way better. Plus all the old heads used Rusto. With stock caps (nozzles).
Bang out them tags

 

Move your body with the can when painting outlines, not just your arm. When you only move your arm your can control gets screwed.