This is a cool way to learn how to draw graffiti in legal areas.
Familiarize yourself with different styles of graffiti. No matter where you live, you can probably find graffiti nearby. If you can't find any in your neighborhood, visit a city if possible and look there--graffiti is predominately an urban art form. If a trip isn't possible, look on the internet. There are hundreds of websites that have pictures of graffiti; you can take a virtual tour of styles from across the country and around the world. Some pieces are obviously better than others, but the more you see the better you'll know the characteristic styles of graffiti.