It can be difficult to get formal instruction in art forms spurned by the establishment, at least until those Rollerblade-hating school boards begin to recognize the importance of also expanding your fishbrain. Because they didn't teach spraypainting at your public school either, check out the 2Kool Graffiti Workshop.
Founded by two spraycan whiz kids who had to perfect their craft while dodging the 5-0, 2Kool's a street artist collective that normally leads art classes for at-risk youth, but are now tweaking things to your more grown-up tastes, vowing to get your skillz street-ready in just over two hours, while focusing more on the art of tagging than the recess-time playing of it. For just 20 bones, the boys will set you up with all the materials you need to learn the basics of graffiti and general street-art (from paints, to stencils, to both canvas and a real wall to practice on), and will start by helping you come up with your own nickname (and corresponding tag), before moving to more advanced techniques like color blending and spraycan control, which can be used to both create shadows, and make the dude from the huffing Intervention do basically whatever you want. They'll also draw on years of experience to wax poetic on "the messages behind graffiti", as well as how to best avoid/deal with the authorities whilst plying your new trade, from evasion tactics like "be prepared with a good hiding spot", "have a lookout", and "stretch out beforehand", to how to handle interrogations, namely: "admit it", which as far as strategies go, ain't too Shaggy.
To maximize fun and creativity, they're also welcoming you to BYOB to class, meaning you'd better reserve your spot quick, as there are only 20 available spots, and anyone who likes booze and art will be aggressively trying to get inline.