Do you know of any local bike shops (LBS) in your area? Some of them might be selling used bikes and the benefit you get is that you develop a relationship with the people in the bike shop (who will help you fit the bike for your legs and arms and flexibility) and because the bikes are used (if they sell them) you won't be spending brand new dollars on something high end.
The reason I suggest this, rather than just saying what kind of bike you should get, is because if the bike doesn't fit you right, Walmart is not going to care. And you might realize that the bike hurts your legs or your back and then you'll stop riding it. And even if you only spent $100 on it, you'll never ride it again and it'll just get rusty in your garage or shed or where ever you put it....
A bike store will help you make sure it fits, so it doesn't hurt your back or your hands or your feet. They may recommend things that you can't afford right now, but you can tell them that. The nicer shops will develop a relationship with you, so that if something you need goes on sale, they'll call you up or they might offer a layaway plan.
Back to the bike itself... I like multispeed because if you're going up and down hills, those gears are going to come in handy (the easier gear to go up the hills, the harder gear to go down hills -- you go faster!) and you can work your way up the gears (usually 3 in cruiser bikes) as you get better at it.
Secondly, have you taken a look at the cyclists out there? This weekend, the Olympics had the road race for men and women (Saturday and Sunday) and those men and women are skinny!!!! Strong, obviously, because they have to pedal their little legs off, but definitely skinny.
Yes, cycling burns off calories, more so than walking and I believe, even running.
I cycle and I do it because it's easier on my knees than running. I can burn 500-600 calories an hour and it feels like nothing.... because I love being on the bike! The sensation of moving fast, the wind in my face, being outdoors, and feeling like I'm part of something bigger.
When I'm in a car, I feel encapsulated, in my own little world. I like feeling like I'm part of a bigger thing and I can do that on the bike. I say hello to other cyclists and joggers on the road, I can hear the insects and birds, smell nature, all that stuff.
It's good for your body and it's good for the soul, in my opinion.