once the printer is turned on. simply lift the lid and the cartridges should park allowing you to remove them by pushing down on the back of each cartridge to release it. they are usually labelled b and c on the printer above the cartridges so you know which one to place in which slot. "b" for black and "c" for colour.
You can generally find any kind of ink at your local Wal-Mart. You can look on the official Canon website, locate your model, and it will tell you the kind of ink you need.
Stores like radio shack, cameras etc., and other stores that specialize in computers/technology will probably have the best deals and be the best quality.
Try to talk to a specific place like Staples that might sell ink and they might be able to help you get a discount since you are a student and you use limited money.