There are generally three things that become apparent with fish illnesses.
1. Ich. Looks like grains of salt. Turn your heat up to 87F, two degrees an hour, and then leave it there for two weeks. No medication is necessary. If the fish looks like the case is severe, you may want to use medication. Any local fish store should carry it.
2. Fungus. If it looks fuzzy or furry on your fish, it's likely to be fungus. Pimafix is available at any local fish store to treat the fish.
3. Bacteria. Seen with things like fin rot, Melafix is a medication available at any fish store as well.
Careful with all medicines as they may be colored and stain silicone sealant.
Pictures would help us identify what you are looking at, but that quick summary should cover most of what you will encounter. Good luck and let me know how it goes.
