Finally bought a bike. Shit, I am so out of shape. I also kind of ate it trying to get on the bike to test it out. Already, my mind is filled with questions:
-how do these weird gear-shifters work?
-why am I having so much trouble getting onto and off of the bike?
-how am I going to get around while never traveling on major roads where insane STL drivers will kill me?
-why didn't this bike shop sell helmets? Small businesses in this city LOVE not carrying things and forcing me to drive out to Target. WTF guys?
1) I'm going to assume since you mentioned paying around ~$10 for the bike you have a steel-framed road bike from the 80's boom. Do you shifters look like this?

Those are down-tube shifters (the diagonal part of your frame is called the
down tube. It runs from the head tube [the vertical tube that your stem and forks run through] to your bottom bracket, where your cranks connect to the axle). They take a little getting used to since you have to take your hands off the bars to use them, but the advantage is they are extremely easy to service yourself, and if they are friction shifters (they don't "click" into discreet positions) they are compatible with pretty much any derailer and cassette (the group of gears on your back wheel).
A couple things to remember about shifting:
- Try to anticipate your down-shift before going up a hill. If you shift under a heavy load (when you are pedaling really hard to get up that hill) your chain will probably skip a gear or several teeth and has a greater chance of coming off completely.
- Try to avoid the gear combinations that put a lot of strain on the chain, especially the "small front" and "small rear" combination. Because the chain is at more of an angle, this puts more stress on the chain reducing efficiency and wearing the chain out faster.
- Attempt to keep your cadence in the same general zone. Around 70-90RPM is usually the most efficient, and it will give you a better aerobic workout than a higher gear would.
2) How did you size your bike? If you stand flat-footed over the top-tube you should have about an inch of clearance between it and your crotch. When you are pedaling, your leg should be
nearly fully extended. If that checks out you should just practice a bit more, especially if you have a road bike and aren't used to riding that style.
3) Use google map's "bike directions" option. I've used it extensively for the past couple years and it is a really fantastic way to learn which of your city's streets have bike lanes, wide shoulders and to figure out where your off-street trails are.
I'd also urge you to check out your city's bike collective on an open shop day:
http://www.bworks.org/bikeworks/They are an amazing way to learn how to do pretty much all repairs and maintenance on your bike yourself. They tend to specialize in 80's road bikes since there is a huge amount of them in the country, they last forever, and they are simpler to tune and repair than a modern road bike.
Good luck with the bike! There's no better way to feel like a kid again than riding around on a nice sunny day.