Learn How to Choose a Bike?
First, obviously, you need to decide how much money you are prepared to spend. I advise being generous, as I said earlier; spending extra money will get you a better bike. Expect to spend at least $250; you won't get many choices at that level so realistically you should expect to spend $300.
If you can't afford to spend that much, you can get good deals on good used bikes. If you look around lots of people never ride their bikes and eventually wind up just dumping them. A used quality bike will serve you much better than a new Target junker.
Once you have a price range, head to a bike store. Do not allow yourself to be rushed; test-ride a number of bikes and don't skimp on the rides (details on test riding below).
Consider theft resistance; does the bike have a quick-release seat that you'll need to buy a cable for, or take with you? Look at the frame joints; are the welds (or brazes) clean or sloppy? Are the wheels true? The brake lever end should have a maximum travel of about 2 inches (a little less for mountain bikes) and you should not be able to bottom it out.
Are the brakes sidepull or cantilever? (Cantilever brakes are mounted on pivots on the fork and seatstays, with the main cable pulling a transverse cable, which straddles the wheel. Sidepulls are mounted directly above the wheel with the cable pulling them together on one side.) Cantilever brakes are good if you plan to put fenders on the bike (which immensely improves cycling in the rain). Sidepull brakes tend to work more smoothly and stay in adjustment better.