You will need to jailbreak it and be sure to "hacktivate" it when you do so. Read the info in the sticky thread and then find a tutorial for the 2G on iClarified.
Read the sticky threads at the top of this forum. You shouldn't do anything until you've read them--all the way through. They will tell you all you need to know. This isn't something you should do based on a two minute post by me.
Since he gave u the 2G iPhone, what FW is on it?
If its 3.0.1, then all you need to do is shift restore it to 3.0.1, then when the activation pops up, just unplug the phone.