You can either call the listing agent directly (should be on the sign or hopefully they have a brochure) to set up a showing. Know upfront that most listing agents require a pre-approval letter from your lender in order to write up an offer, so if you have not already done so you need to get working on the financing side.
If the foreclosure is on Trulia the listing agent's info should be right on the property description.
Otherwise Trulia has a directory of Realtors in your area. The beauty of Trulia is you can research the realtors right on this website and see how they work on the website. In Minnesota the seller pays our commission so you are getting a lot of service for nothing.