Ask for, and check, references. Until you've selected an agent, let other agents you come in contact with know that you don't have a representative. You could, for instance, find the right agent for you at an open house. Also, once you've committed to working with an agent, let other agents know that you have one. The next step is to talk with a mortgage broker or loan agent to find out what price home you can afford.
Some buyers like to start the process by finding out what they can afford before they pick an agent.
That's fine, but a benefit of choosing a real estate agent first is that your agent can put you in touch with loan agents. Preapproval is not the same as prequalification. Prequalification, which can be accomplished by simply calling a mortgage broker or lender, only tells how much money a lender is likely to loan you. It's not a guarantee. Preapproval, on the other hand, is a formal loan-approval process.