+1 with chewybrian. Don't get a wal-mart bike; they are generally very low quality and badly assembled. If you ask people here which wal-mart bike to get, you'll get the answer "none of them."

If you do get a wal-mart bike, follow these two simple rules:
1) no suspension,
2) no gears.

That's because it's very hard to mess up a single speed bike with no suspension. You still won't get a good bike, but you probably won't get a bad one.

There are some good books at your local library on buying and keeping a good bike. It's worth a quick trip down and a question to your librarian. They're generally fun reads, too.