The latest firmware available for the WRT54G is available on the Linksys WRT54G Downloads page.
Important: Be sure to download the firmware that matches the hardware version of your WRT54G router! The hardware version number can be found on the bottom of your router.
Here's a direct link to the Linksys WRT54G Manual, which is PDF format.
Everything else you could possibly want to know about your router can be found on its support page on Linksys' website: Linksys WRT54G Support.