guojia38,
One way you can save some money is to look for a frame that is made of cro-moly (or cromoly). It's a type of steel. It's a little heavier than aluminum but has a smoother ride.
Don't skimp on quality components. Stay away from anything that doesn't have Shimano or Sram components. Many entry-level bikes have Shimano Sora components. While they're not the best Shimano has to offer, they are reliable and will last a good long time if taken care of and not abused.
Good luck
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