There are tons of tutorials on youtube. Just search how to graffiti, how to draw graffiti, how to do graffiti throwups (pieces, handstyles), etc..
The main objective is to develop your own style and be able to draw out your ideas and visions on paper. So what's the real solution to that problem? Practice. Practice, practice, practice, practice, oh and did I mention PRACTICE! Haha seriously practice works wonders. Surround your self with graffiti and/or just plain old drawing throughout your day.
EDIT: Just finished coloring one the drawings I posted above with my new shipment of prismacolor markers. Donald this is dedicated to you haha they really are probably the best markers I've used so far.
EDIT: Just finished coloring this one, I did not want to put that star there but I had to because my paint marker leaked right there as I was white lining so I had to make something out of it. Grrrr.