Good idea for a new hobbyist to go out on the web and look up 'marine ich', 'lymphocystis,' 'gorilla crab', 'fireworm' [rare, and not to be confused with a bristleworm, which is a good guy]. Look until you see pix. If you follow good practices of quarantine and careful water quality, etc, you may NEVER be troubled by such. But they're good to know by sight, and best not learn the id in the school of been-there done-that. If you can correctly id the problem, someone here can tell you what to do; if you can spot the problem, you can avoid buying a sick fish. This will save a lot of grief. Do note that in the case of ich, some fishes have a thick slime coat and will pick up tiny grains of sand that LOOK like ich, worrying the owner. Just observe over several hours. If the dot goes away it was likely just a sand grain.
