Proper Bike Fit
Price range dictates a range of "this will do" all the way to "custom bike made just for you". Where ever you fall on that scale, the better the bike fit, the better your experience will be.
The first and most basic step is to pick the right size frame, and that is done by standing over the bike, aka stand over height.
Stand Over Height
Standing over the top tube, lift the bike evenly off the ground by the seat and handlebars. You should have an inch before it touches anything. More than an inch and the frame is too small, less than an inch and the frame is too big.
Measure the distance from the center of the Bottom Bracket to where the top of top tube intersects with the center of the seat tube.
This is done so that the manufacturer doesn't have to make a frame for every inch / cm, instead they only have to make a few frame sizes, and then size the bike using the components. On a bike like this, you should have 3-4 inches above the top tube.
Sizes: S, M, L
Horrible name, because if anyone should be riding these bikes it is boys, but it was dubbed during a time when dresses were more common and lower top tubes made more sense.
As far as sizing goes, imagine where a boy's top tube would be on a girl's bike frame, that is what you use to determine the correct size.
Here is a rough guide of age to bike size. Note that kids obviously vary in size A LOT, so this is a rough guideline using wheel size with some overlap...
- 12" for Ages 3-5
- 16" for Ages 4-6
- 20" for Ages 6-9
- 24" for Ages 8-14