Hi Yongzhi, you are about to get a number of agents responding to your question that will be offering their services, myself included.
As a Realtor, Mortgage Broker and Certified Mortgage Planning Specialist I would highly recommend your very first step be a Pre-Approval if you desire accuracy in knowing your true purchasing power and total costs upfront. The type of financing you choose, whether FHA or Non-FHA financing, has an immediate effect on what your purchase options are. There is a difference between a Pre-Qualification and Pre-Approval. In short, a Pre-Approval is a verification of purchasing capability. For the differences between the two see: http://www.Steven-Anthony.com/default.aspx?pp=39377
Via the Pre-Approval process you will know exactly what you can "technically" afford based on current interest rates. Then, it's up to you to decide how comfortable you want your payment to be by reducing your target sales price or increasing your down payment. MOST IMPORTANTLY, the Pre-Approval makes you aware of any issues that will prevent you from buying when you want to.
As part of the Pre-Approval you should also ask for a Good Faith Estimate if you want to know all of the costs of purchasing a home based on a set Sales Price that includes all third party costs (title, escrow, insurance, property tax, appraisal, inspections, etc.).
Once you know what you can afford, you can then focus on your needs and the other research about where you would like to live. There is much more to discuss once you choose your Realtor. I would be happy to meet with you to talk in more detail.