Each of the major brands sell nice bikes in the $500 price range. Your satisfaction with your bike depends FAR more on the quality of the bike shop than the brand of the bike. Visit the two or three shops that are closest to your home. If a shop is six blocks from your home, it is easier to stop by for adjustments than if the shop is six miles from your home.
A good shop makes sure the bike is the correct size, that the seat and bars are adjusted to fit you, and that the wheels are "trued" and the braking and shifting carefully tuned. In a month or so, after you have put some miles on the bike, the shop will check over the bike and make any adjustments that are needed.
Try to ride every day. Riding just once a week will leave you sore and tired, while riding every day will soon have you feeling ten years younger.