Margaret has some very good advice. I'd recommend going to your local bike shop (LBS) to find a bike. They will be a little more expensive than Walmart (probably) but they will also help you find the right bike so you'll be more likely to ride, help you learn to operate your bike (shifters, brakes, etc), help you learn to take care of your bike so that it stays in riding shape, and help you fix it at a reasonable cost (often for free if it is minor) if something goes wrong--at Walmart you are on your own for all of that. Even for light riding, chains gum up, brake cables stretch, and tires get punctured and go flat, and if you don't have a resource for help, no more biking! The LBS will also know about nice trails, riding clubs, things like that in your area.
Long enough have you dream'd contemptible dreams,
Now I wash the gum from your eyes,
You must habit yourself to the dazzle of the light and of every moment of your life.
-from Song of Myself, Walt Whitman