There are basically 4 kinds of bikes: road, mountain, hybrid (a hybrid of road and mountain), and comfort.
Do you want to bike on roads, trails, sidewalks, or a combination? I have an old (mid-80s) road bike that I LOVE to go on roads and paved bike paths with. It's fast, fun, and gets me to class on time! Comfort bikes (or cruisers) are good on paved surfaces, but don't go as fast as road bikes. They're a more upright seating position though, if that matters to you.
If you want to go on anything unpaved though, like a gravel trail or road, or a dirt trail, you're going to need a hybrid or mountain bike. They will have tires that are wide enough to handle that kind of terrain. The wider tire makes them more steady, and they can still be pretty fast. Mountain bikes also have suspensions to deal with bumps. Most of the bikes at Walmart are mountain bikes.
A lot of people around here bike to work, and the most popular bike for commuting and occasional recreation is a hybrid bike. They go fast like a road bike, but are more stable with wider tires, like a mountain bike.
My opinion is that if you get a well known brand of bike, it should last you a long time. The Walmart brands will work for a few years, but a $300 bike now will last you a lot longer, especially if you don't bang it up. My bf has a Schwinn from 69 and although it has its problems, it still goes pretty well. Paying more than 300$ at a bike shop is pretty much par for the course.
Things to consider about storing it outside: some newer bikes have "quick-release" wheels and seatposts. This makes them easier to take apart to put in your car, but also a lot easier for people to come along and steal your wheels or seat. You'd need to make sure you use a sturdy lock (the Kryptonite U-locks are great) and loop it through your front tire as well as the frame at the very least. You can get a separate small lock for your seat. Some cities have a bike registration program where you purchase a registration for your bike and if it gets stolen it's a little easier to track down. Bikes are really hard to track down when stolen, however, especially because they can be taken apart for parts. Also, when seats get wet, they are not fun to sit on. Most new bikes won't rust, especially those from a bike shop, but you'd have to be sure your chain is well-oiled.