As long as you have an iPhone 3GS or newer, all it takes is an upgrade to your iPhone's data plan - specifically, to the 4-gigabyte per month plan. This data plan is $45, or at least that's how much AT&T is charging me. (Verizon's price may differ). Once you have upgraded, going into Settings on your iPhone will then have an additional choice called "Tethering." Turn Tethering on.
On the iPad, go into Settings --> Bluetooth, and you'll see your iPhone come up in the list of available Bluetooth devices. Tap on your iPhone in the list, and follow the on-screen prompts to complete the syncing process.
When you are done using your iPhone's data connection on your iPad, go back to your iPad's Bluetooth settings page and turn Bluetooth off, or else it will continue draining your iPhone battery. Tethering is quite a battery hog. But it works great - I use it every day, more or less.
If your iPad 2 does not access 3G, you will not be able to access 3G networks through your iPhone connection. The iPad 2 does not come equipped with 3G but it comes equipped with Wi-Fi so that you can browse the internet through Wi-Fi signals.