To begin with, the most important thing to remember about your agent is that they are almost certainly working on commission. This fact can affect the buyer-broker relationship on several different levels. Essentially, you don't want to waste your or your agent's time. If you're not sure that you are ready to buy or if you are just looking, that's fine. Just be upfront with your agent to ensure a good working relationship.
Be prepared to sign a buyer's broker agreement. Consider the signing of this agreement as the formal beginning of your home search with your agent. Be sure you understand the difference between an exclusive and a non-exclusive buyer's broker agreement: generally speaking, with an exclusive buyer's broker agreement you promise not to hire any other agents to help you find a house. Conversely, a non-exclusive agreement allows you to hire other agents.
Let your agent know if you plan to go to an open house, as some agents prefer to accompany their clients. While browsing open houses, you may encounter agents who are looking for unrepresented buyers. Always be forward with the fact that you are already represented in order to avoid confusing another agent if you are already in an exclusive relationship. If not, feel free to ask any agents you meet at open houses for advice and they'll be willing to help.
Finally, understand your agent's obligations to you. Be very clear what you're looking for in a house. Tell them the best way to contact you when they find a house they want to show you. Absolutely never sign a contract that you do not understand. It's an agent's job to explain all of these agreements in a way that clear and easy for you to comprehend, and most will be more than happy to take the time to address any concerns or questions you may have.