The store you buy your bike from is responsible for assembling it. A poor assembly job can make for a very poor riding bike. Make sure you feel comfortable with the competency of the staff. You want a store that you will feel comfortable brining your bike back to for work in the future. The Right Bike.
Test ride a variety of bikes.
Try bikes in the low end of your price range and the high end and feel the differences. Try bikes with different components to see which you like best. Larger stores have more of a selection and are also more likely to have a model in your size available for a test ride. When you take a bike for a test ride, try to take it out over more challenging terrain than the parking lot. Ride over curbs, gravel roads, grass, etc.