Have you observed the graffiti downtown? If you haven't, then take a peek this weekend and look closely to find out what I mean about self-expression. After checking it out, you'll see that graffiti is not pure vandalism after all. In fact there are organizations or groups of people who became well-known because of graffiti and are now paid well to create it.
One such group is the TATS CRU in New York who are paid to paint on street walls, subways and streets. Tats Cru claim to be "The mural kings who have changed public opinion of graffiti as an art form and have taken it to new heights." It is no longer seen as vandalism.
If you want to learn this art form, start the easy way to become a graffiti king. Grab a large piece of paper-preferably plain newsprint which is cheap, use a piece of charcoal or graphite pencil, and start with a few letters all in caps. Keeping it large will help you when you get to that huge wall that looks inviting!