I think you can do a whole lot worse than buy a Diamondback bike as an entry level ride. Dick's Sporting Goods sells them and probably some other sport retailers too. They are far better than anything you will get at a Walmart, Target or other department type store.
This week, Dick's has bikes for 1/2 off, so now, a Diamondback becomes more affordable and competitive with the better bikes at Walmart and Target.