Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Hard to say much since you give little or no info pc/mac firmware version. however the 1g is the easiest to jailbreak probably going to use Redsn0w and look here http://wikee.iphwn.org/howto:rs9 tutorials here http://www.iclarified.com/tutorials/iphone/index.php or youtube Bing or Google it really simple. stickies up at the top will help and I'm sure there are a 100 threads if you search. easiest way though is a non activated sim from that carrier say att. then reboot. you have to jb then unlock.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.