First, grab yourself a copy of redsn0w (Mac and Windows) here.
Insert an IPSW in redsn0w (must be the same as the device being jailbreaked). Choose your options. (For iPhone 2G users, here are the bootloaders.)
Turn your device off (either by holding the lock button or holding the lock and the home button together).
Press next, then it will have you press the power button for 3 seconds and with out releasing it, hold the home and power buttons for 10 seconds, then release the power and continue pressing the home. After you see activity on redsn0w you can release the home button.
If you get hanged on redns0w and with it saying “Waiting for reboot”, simply unplug and replug your device in your computer and it should do the trick.
And welcome to the Jailbreak community!