How to tether the iPhone 3G and browse the web on your laptop
First, you’re going to need an iPhone 3G that’s been jailbroken, and two things called 3Proxy and Terminal, plus a laptop of course.
To jailbreak your iPhone use the recently released PwnageTool for Macs. Then when you’re finished with the jailbreak, dig through Cydia to install 3Proxy and Terminal on the iPhone.
There’s a very detailed step-by-step how-to written on Nate True’s blog, so I won’t serve any spoilers and advise you to check that out instead.