My advice is to look for a sale at a LBS or consider getting something used. I see decent used bikes selling for less than that department store junk all the time on Craigslist. Take some time to educate yourself here and elsewhere on the internet for what to look for in both used and new bikes.
I know some may disagree, but IMO going with a department store Diamondback or Schwinn is a false economy and if you actually like biking, will wind up costing you money.
Edit. Meant to say buying a department store bike will, in the not very long run, cost you more than had you just bit the bullet and bought something decent to begin with. It isn't that entry level bike shop bikes are all great bikes, but they are at least good or adequate bikes that should last you for years, and will be backed up by a warranty from a LBS.
Last edited by MRT2; 09-02-13 at 06:15 PM.