What to do if I forgot lock code

You can restore the iPhone from backup on the last computer your iPhone synced with, this will allow you to reset the passcode http://support.apple.com/kb/HT1414. If you do not have access to the last computer the phone synced with, you will have to force the phone into recovery mode & restore as a new device, all data on the phone will be erased http://support.apple.com/kb/HT1808.

You must restore using the computer you normally sync with. If that computer is unavailable, or you did not set the passcode before your last sync, then you will have to put it in DFU mode and restore.