If you're running firmware 1.1.1, simply go to http://jailbreakme.com on your iphone to install AppSnapp from which you can install summerboard, which then lets you install themes and such.
If you're running firmware 1.1.2, there's a few more steps you need to go through. If you're running 1.1.2 and would like to jailbreak your phone (install app snapp and themes), shoot me a PM, or add me to msn or aim and I'll help you out.