I forgot my passcode

Boot your iPhone into recovery mode and connect it to your computer to restore it from iTunes

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.