These links encompass a LOT of useful knowledge. Search through these FIRST; there are few questions they don't answer.
VWVortex Mk1 TT FAQ - Covers many common problems.
VWVortex 1.8T FAQ - Covers a lot about the 1.8T engine including troubleshooting common issues, performing routine maintenance, etc.
AudiWorld Mk1 TT FAQ and Common Problems HUGE listing of links to threads.
Audi Forums Mk1 TT FAQ - Scroll down and you'll find a post with links to a LOT of how-to's.
VWVortex Mk4 Volkswagen FAQ - Don't be too snobbish to go here. Your TT has a huge amount in common with Mk4 Volkswagens, there's good info regarding engine related stuff in this read.
Jeff Bipes' TT Site - Lots of useful writeups by Jeff.
TTForum.co.uk FAQ - Not very big and from the looks of it not too useful, but I thought I'd include it anyway.