How does the law of agency work with using online listing agents?

[Edward A.]

Just looking to get some general advice. I'm looking to purchase my first property and have been mainly using realtor.com in my search. As most of you are aware, each property on the site has their own listing agent you can contact to get more info. 

So I contacted an agent, and after some chit-chatting, it turns out that the property in question probably isn't the best investment for me. No biggie. But the agent is of course willing to help out with my house search and has recommended other properties for me to check out.

As far as knowledge of the area and experience with investors, she seems like a good person to work with. But how does the law of agency work here? Is it safe to assume the agent will have my best interests and will essentially act as my buyer's agent? Should an agreement be signed between us before looking at properties?