Welcome to Trulia.
Most of the listings on Trulia are syndicated from other places you may already have input your listing (i.e perhaps your broker's website, property website, syndication vendor, MLS...).
If you want to enter your listing manually on Trulia please click on the following link and go through the steps:
Good answers below, just want to add a few details. You may want to check and see if your listings are already on Trulia. To do this, log into your account and go to your listings page. From there you can do a search for listings based on address, MLS#, or your email address (if that is fed to us with a listing). I think many agents are surprised to find their listings already on Trulia. As Catherine stated below, you can submit listings directly on Trulia as well, but you may also want to use a third-party to syndicate your listings. The link below will allow you to see all your options, including some of our main syndication partners. Good luck!
Account Manager | Trulia