how can i unlock my old iphone 2g

The original iPhone was only sold locked to AT&T. it's pretty much completely useless now. There isn't a US carrier left with a 2G network and most carriers around the world that did have them are shutting them down to re-use the bandwidth for higher speed networks.

Carrier locked? You contact the carrier it locked to and have them unlock it, then your restore the device as new in itunes.

If this isn't what you mean, then please explain in details.