I will assume that you've already decided that a canary is the right pet for you and you understand their basic nature and requirements. First, you need to decide how many birds you will get and what gender. Most people want a male canary because males sing. You should get one cage for each male because they are very territorial and will fight with each other. If it isn't breeding season, they may even fight with a hen.
Some people prefer hens, because they are more sociable.
You can put several hens together in a flight or aviary. It is a common mistake to buy a young male and then discover it is actually a hen. Most people get attached to the bird and are reluctant to return it when they discover the mistake. Since males are more expensive, ask for a written guarantee when you buy a male, especially if buying from a business. If the bird does not sing within two weeks, return it.