I'll second DataJunkie on that. A Wal-Mart bike can range from substandard at best to dangerous at worst.
And while there are some awfully trashed used bikes out there, a decent used bike will serve you better now and down the road than a Wal-Mart bike will.
I bought a Target bike in HS (yes, scoff if you will, but teenagers are stupid, and I didn't know better). Within a couple of weeks less than half the gear ratios were useable, as the derailleurs wouldn't make the appropriate transitions.