How to choose a bike that you will be happy with, when you're still growing
I am often asked, mostly by people who have never ridden a bike before, how to choose a bike. While there is no simple answer to this question, there are some basic things to keep in mind.
Rather than writing everything that someone else has already said, it's easier to direct to another page, so if you are wondering how to choose a bike, here is some information: The Beginners Guide to Picking the Perfect Bicycle.
Now that you've read that article, a few add-ons: I generally do not recommend anyone buy a bike from places like Canadian Tire, Walmart or Costco. These bikes, especially the kids bikes, are made with heavy steel frames (which will rust), cheap parts (shifters, brakes) and are really not worth the money. Bikes from sporting stores like SportChek are better, though some are still steel frames, as the people who assemble them are usually bike mechanics/technicians, but sometimes they cost more than they are worth and their parts may still be lower quality.
Also note that the prices in this article are American: if you are looking for a child's bike with an aluminum frame and good components, expect to pay at least $400 in Calgary, but keep in mind that with proper care, you shouldn't have to replace the most expensive part of the bike (the frame) ever and all the parts will work smoothly for many years. You may never have to buy another bike again (but do expect that parts, like the tires, chain and cables will wear out) and when it's outgrown, it can be passed down to younger siblings/cousins or sold - good bikes don't lose their value as quickly. Our choices here in Canada are a little more limited, but there are still awesome bikes easily available. There are also options for buying used bikes that don't involve driving all over the city or interacting with strangers.
That being said, there are fewer choices in style of bike for kids. If you are a younger participant, you may have limited options. Bikes with wheels under 24" diameter are pretty much the same. Once you get a little older, and bigger, more styles become available.