This sounds great! First off, there's a general welcome page for contributors, which you'll want to read: http://www.eclipse.org/contribute/
I'm sure every project and component under the Web Tools (and likely, every Eclipse project) can use qualified help.
If you have a special interest in one of the sub-areas of WTP (web application support, XML/XSD work, etc.), you could start there by searching for open enhancement requests or bugs, using the Bugzilla system .
Another thing you can do is to review the plan for Kepler, the
release coming in 2013 . There you will find links to features being worked on.
Also, if you have an idea for a useful feature, please open an enhancement request in Bugzilla, and describe your idea, and the relevant committer will assist you.