Step 2: The Single Most Important thing to know when buying any bike (new or used)
Ever. Don't even consider it. Seriously. Not even for your kid. They are absolute garbage. They shouldn't be legal to ride on public streets. They should be considered toys at best. This includes pretty much anything you can buy at WalMart, Target, or Toys'R'Us, or whatever local equivalent you may have where you live. The most common brands are Huffy, Murry, and Magna. Generally anything branded with a car/truck name (GMC) is one of these also. One of these bikes brand new is worth less than a real bike that is 30 years old. (Actually, my favorite bike, the one that I rode to Mexico and used a messenger is about that old).
I can not emphasize this enough. Having worked on these many many times, I can say with absolute certainty that they are not worth the price if you are given one for free. They use the cheapest parts that can be found, parts which have been obsolete for decades in the rest of the bike world, and those parts are put together by people who know literally nothing about bike mechanics. I have rarely seen one, even brand new, that had everything adjusted properly. They are so cheaply made, and so poorly assembled, that they are dangerous.
*I have already gotten a couple of comments suggesting I must be a "bike snob" to say this. Not so. All of my bikes have been cheap. I have never been a racer. The shop I worked in was a non-profit community bike shop, and we never turned anyone away, no matter what they rode. That's how I got to see so many department store bikes up close and from the inside, as well as test riding them (after repairs).
These bikes are not suitable for casual riding, and not just because they are heavy. They are built with the cheapest parts possible, and then poorly assembled. By riding one you are automatically increasing your chances of getting into a crash.
From a mechanics perspective, the poorly built parts translates to additional labor for repairs (which they need more often) which in turn means higher repair costs - often higher than the original purchase price. For this reason, many bike shops will not even work on these bikes.
The best clue that the bike you are looking at is a department store bike is if it has a one piece crank.
See pictures above for examples
Another clue that the bike you are considering is a rolling pile of crap is brakes that look like the ones in the pictures, called simple caliper brakes.
Note that most road bikes will have compound caliper brakes, which are similar in shape, but slightly more complex. The keys to the crappy brakes are the rectangle brake pads and the flat metal brake arms, with just one single pivot point in the center.