Check out other graffiti styles that artists are using in your town, or look online at graffiti.org for the types of graffiti that people are creating around the country. Keep track of graffiti you like in your sketchbook.
Develop your tag, the basic part of a graffiti artist's repertoire. It usually is a stylized drawing of the artist's moniker. Practice your tag several times in your sketchbook. Try different shapes for the lettering until you hit on something you feel is uniquely your own. When you get to that point, practice the tag until you can deliver it quickly and precisely.
Develop larger scale projects. This starts in your sketchbook. Keep a careful track of all your ideas, and use the sketchbook to develop color schemes, geometric takes on your tag, drawings of characters that you like, and more.
Expand your skill set. Graffiti artists use stencils, wheatpaste posters, stickers to put their messages out there. Many graffiti artists use photocopy machines, projectors, or digital imaging software to aid in their designs. These are all tools that you can master as you develop your own graffiti style.