Before you take a step further, you need to know a few things:
1) Neither iPhone modding, not unlocking are permitted by Apple at this time,
and BOTH WILL VOID YOUR WARRANTY if Apple finds out.
2) "Modding" generally refers to changing the look and feel of your phone, and/or adding 3rd party software that is not approved by the maker of your phone.
3) "Unlocking" generally refers to "Carrier unlocking", which makes your phone usable on any carrier that supports your phone type (in the iPhone's case, that is GSM).
4) Most of the software modding that you can do to your iPhone can usually be reversed by using the Restore feature in iTunes.
5) The unlocking process is often permanent.
in case you missed it ealier...
MODDING AND/OR
UNLOCKING WILL VOID YOUR APPLE WARRANTY
Proceed at your own risk.