1) switch the printer on
2) lift up the scanning/copy platen - put you fingers underneath the LED display and lift it
3) in a couple of seconds the cartridges will move over to the left, where you can pull them out and replace them.
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.