Personally, I wouldn't go to Kmart, Walmart, or Dicks for a bike. They're all basically the same models with different paint. They are mostly the lowest-end models from Pacific-Dorel Bike company. You'll notice the cheapness of construction right away - plastic cranks instead of metal, for example.
Check a local bike store in your area - I hope there is at least one - for some better brand names - Trek, Specialized, Cannondale, and other "chain" brands are a huge leap in quality. Where are you located?