There are lots of projects on the tt-forums. They all have their own topic. Just post in the forums that you would like to help, and they would be very happy, I guess. (Note that DBSet isn't a forum project, if you want to help with DBSet you should contact Michael Blunck.)

Starting a new project is possible too. Most projects are train sets with trains from a specific country. The problem with starting a new set is finding one or more coders to help. But when you start a new topic on tt-forums, you probably get some reactions and you might get a coder and more artists to help drawing. (Even if you aren't an artist, you can start such a topic.)

Before starting a new project, you must check if there wasn't already someone who posted a topic for a similar project .

REVISION
Sets usually start out with a Tracking Table. Such a table includes a list of vehicles to be included, with their pictures (for artists) and statistics (for the coders). Putting a Tracking Table at a website would make it easily available for everyone.