Go to several local bike shops in your area and test ride all the bikes in your price range. Find out what is comfortable and what's not. Things you like or don't. For example, rapid fire style shifters versus twisting grip style shifters. You may even get lucky and run across a bike that is last year's model for a discounted price. Don't be afraid to ask if they have any older models hiding the back, two of my local shops did and didn't hesitate to let me ride the bikes and throw me a price that was significantly lower than the current year's model. As was stated, buy a helmet. Wear it. Make sure it fits properly and is comfortable, otherwise you'll be tempted to throw it to the wayside.
Kris