I think what you're trying to ask it to "unlock" your iphone 3g. If you bought it from a provider that sells it locked (say you bought it from O2 in the UK), but you want to use a different carrier (like T-mobile or 3) then you would have to unlock it.
To unlock it, you would need to "Jailbreak" it first, then unlock it. The tools for both of these were developed by "Dev-Team". You have to be careful though; Jailbreaking or unlocking your phone may void your warranty. If you are not tech-savvy but still want to do this, I suggest getting a friend to read through the instructions, then doing it for you. If you do not need to use your iPhone with another carrier than the one you bought it from, don't even bother doing it. Jailbreaking a unlocking is best left to a bit more rechnologically involved users, not that it's difficult, it's just those are the ones that are likely to benefit the most.