I'd strongly recommend buying from a local bike shop. Actually, I'd recommend visiting several shops until you find a sales person who you feel comfortable talking with. One of the best services that any decent bike shop will offer is helping you to pick the right size. You wouldn't buy shoes that are too small just because they only cost half the price would you? Bikes are the same way. A right-sized bike will be much more comfortable, efficient and fun to ride so you'll be much more likely to want to ride it.

Besides the bike I do recommend wearing a helmet. I'm guessing that you'll want an aftermarket saddle too. Each of those things are going to cost in the $35.00 range so your $300.00 budget might be a little unrealistic. Compared with health maintenance costs, however, an extra $200.00 is nothing.

Walmart bikes are for people who don't understand the difference between features and quality.