Choose one of the walls at the top by clicking on the numbers, find the one that could best use your personal influence!
Once you've found the wall that you'd like, just click "Tag!" and start to improve the scenery! Click the arrow on the left side of the screen to bring up your personal palette of spray paint choices. Remember that you can only spray a limited amount before you either submit it with "Done" or scrap it with "Reset".
Once you submit it, feel free to start the process again and continue to improve your work.