OK, your gonna need a graphics program like Fireworks or Photoshop (I'm gonna show you in fireworks cos it is the easiest to get hold of for
free )
You'll also need to download some decent graffitti fonts, try
www.acidfonts.com for some cool ones.
I've also zipped some of my favourite graffiti fonts
here.
OK, download all the stuff your gonna need and install the software / fonts.