There are a lot of guides online offering pointers to using Thunderbird with Gmail, but my hands-down favorite is the MozillaZine Knowledge Base article on this subject. Besides offering expert advice and tips, the guide is regularly updated to reflect Google's latest changes to its "beta" Webmail service. (According to MozillaZine, at the time of this writing the guide was last updated on Nov. 28, 2008.)
If you're simply logging into Gmail over a secure HTTP connection, then the setup process really is drop-dead easy. The POP and IMAP options (Google offers both for free) are a bit more involved, and they require some setup on both Thunderbird and in your online Gmail configuration, but even non-technical users will find it easy to step through the process with a little guidance.
As Mozilla's guide points out, Gmail's IMAP support is maturing rapidly, although there are still a few rough spots. Its POP support is quite reliable, however, and a POP-enabled Gmail account really delivers the goods.
When you read through the MozillaZine guide, by the way, be sure to check out the pointer about using Gmail with disposable email aliases -- a great way to catch and filter spam before you ever know it's there.
You can find the MozillaZine guide to using Thunderbird with Gmail right here. You might find their Thunderbird configuration guide and advanced IMAP account setup guides helpful, as well. On the Gmail side, you can find instructions on setting up POP and IMAP access here and here, respectively.