1. Not familiar with that particular canister, but the procedure is pretty similar for all of them. Drain a bucket of tank water. Clean the filter media in the bucket. Pads should be replaced if they're hopelessly clogged or falling apart. Bio-media, ceramic rings etc. can be put in a kitchen strainer and swished around in the bucket so the mung and crap falls thru the strainer but the media doesn't go all over the place. The key is making sure you're not washing the stuff with chlorinated water right out of the tap, or water that's too hot or cold. And you want to do it as quick as you can, so it's not sitting around drying out.
2. Prime works great, just follow the directions on the bottle and you'll be fine. I like to use oral dosing syringes when measuring anything I put in the tank. They're only a couple bucks, available at any pharmacy, and they're accurate down to the ml.
3. As MFKers, we tend to be biased toward cichlids. Some of us hear 20g and immediately think...African cichlids! Convicts! But both would be a poor choice for a 20g. Personally, since it's for a biology class, I would be thinking bio-diversity. I would go with one or two types of schooling fish, tetras or rasboras, otocinclus catfish, red cherry shrimp, and live plants. You may have to drop a few bucks on a better light, but you will look like a superstar if you manage to grow some nice looking plants when your classmates think that fishtank decoration means no-fishing signs and deep-sea diver air bubblers.