Well, the other posts gave you an idea of how general your question is, but here's the basics: what type of tank do you want? Reef? (reef tanks have corals, anemones, clams, etc.) FOWLR? (Fish Only With Live Rock) exactly what it means and FO (Fish only)? Reefs tend to be prettier, if maintained right, but you need super strong lighting. Fish only and FOWLR tend to need protein skimmers and heavier filtration in general, unless you have the same amount of fish in a reef tank. With the right amount of fish and filter feeders, disease can be easier to control in reef tanks, but fish, no matter where they are, are susceptible to disease. I would recommend looking for fish you like, and depending on if they're reef safe (won't eat corals, shrimp, crabs, or clams) you can narrow down your question. Keep asking, it'll help a lot, and you're choosing a great tank, the bigger the more stable!