1. Ride the bike around the store before you buy it: To me, if the bike doesn't even ride correctly when I'm in the store, there is no way that I'm paying money to take it out of the store. Specifically, before you buy a bike from Walmart or any of the other big box stores, check to see if a) both the front and rear brake work correctly and b) the bike shifts through all of the gears on both the front and the back. By far the easiest way to check this on a bicycle is to get on the bike and ride it down a couple of aisles. If it doesn't pass muster, look at another bike.
2. Take the bike to a shop and get everything checked and tightened: Before you ride the bike any further, I would take it to a local bike shop to get everything tightened and checked. The employees who assemble bikes for Walmart are usually the same people who assemble the patio furniture, meaning that they don't understand how a bicycle works mechanically or how tight they should tighten the bolts. The adjustments will likely only cost around 20 dollars or so, and it will save you from having dragging brakes, misaligned gears, etc. later.
3. Get the tires upgraded to road/hybrid tires: If you don't take your bike off-road at all and you want to spend about 30 or so dollars more, you can get the tires upgraded to road/hybrid tires. This will literally save you probably about 10-20% off your total trip time if you're riding on the highway or sidewalk. In fact, this is the easiest upgrade of which I am aware that you can make to a cheap bike that will make a significant, immediate impact on your commute time for a novice or average rider.
Well, I hope that this gives you some starting points for buying and making the most out of cheap bikes from a place like Walmart. Good luck riding!
