I am a real estate web developer. There are a few options available to you!
Deborah has pointed out two of the easiest options below. These are virtually free solutions.
If you have a custom web site already, it may be possible to add a script to your site that generates an XML file of your listing data that Trulia needs to "automatically" import your listings into the trulia.com database. Trulia can also "crawl" your web site, to add listings, but this is not the best way to do things.
I'm happy to answer any questions you may have without a sales pitch. I am a licensed California Realtor and have been in the Information Technology side of the business for the past 9 years. I can help point you in the best direction.