Selecting an agent
You want an agent who is experienced and works only in the real estate industry full time. Once you find a home you will be depending on this agent to negotiate for you, and to lead you through the entire process of inspections, financing, repairs and closing.
It is great to have an agent that has earned the ABR designation from the National Association of Realtors as I have. An Accedited Buyers Representative will have special training and will have completed required course work and demonstrated experience as a buyer's agent.
You will want an agent who has time to work with your specific circumstances, for example, if you need to find a home in a weekend, you don't want to buy for a year or two, you'll buy when you find what you are looking for, even if it takes a year or two, etc.
You want an agent you can communicate with, directly and honestly, and who will listen to you.
If you have done a good job of selecting an agent, there is no reason to work with more than one (unless you are looking in a broader area than that agent handles). The inventory of homes is the same for every agent, and if they listen to you, they will plan on showing you the same homes. Why explain what you are looking for over and over? A good agent will sense what you want after one or two times out with you. The agent will also be more aggressive in looking for homes for you if she knows that she is the only agent you are working with.
Once you are comfortable with an agent, you will be asked to sign a buyer's agent agreement, which outlines the period and terms under which you will be working. This protects you and the agent's interest and helps you understand how agents get paid and what to do when visiting open houses and for-sale-by-owners.