There is a mailing list for TextMate. The archive is public and you can search it with this form:
There is a ##textmate
IRC channel on freenode.net.
David Powers has created a succinct cheat sheet (direct link to PDF) which you can print and have next to your computer. It lists key equivalents for many useful general actions.
Soryu has written two TextMate tutorials dealing with setup and basic usage.
You can subscribe to the screencast RSS feed via iTunes.
Here is a list with direct links to a few of the screencasts in recommended viewing order:
For the full list with a brief summary see the online screencast page.