The best advice is get the biggest tank you can squeeze into the space you have in mind. It is a big problem with fishkeepers - we are never satisfied with what we have and always want more and bigger
Tang and clownfish are marine fish. Yes they are tropical, but they require salt water. This is more complicated than just filling a tank with water and getting fish. The water must have additives which must be carefully calculated and monitored. This can be quite complicated for someone who has never kept fish before.
Freshwater tropical are much easier and there is probably a lot more choice when it comes to fish. There are also coldwater, freshwater but the choice of fish is much more limited. Trade in cold water fish is more heavily regulated as they can survive if they get into our waterways, and risk taking over the ecosystem. Cold water fish are pretty much just goldfish - which need much bigger tanks than you'd think.
At the top of the forum is a menu bar. Click on fish profiles and select fish at random. These are all tropical freshwater fish. It will give you some idea of the kinds of fish available, unless you really do want to go down the marine route and get tangs etc.
The size of the tank will limit the amount and size of fish you can keep. But it is not just a case of saying you can have x number of fish in this tank, and y number in that tank. The shape is also important. Some fish dash around and need a longer tank than fish that pootle around. Some fish need to be kept in groups, other are fine on their own or in pairs; some cannot be kept with others of the same species. And you need compatible fish as some will eat others.
Your tapwater also comes in to play. There are fish that like soft water and those that like hard water. Your water company's website should help here. There is usually something on there which will tell you if your tapwater is soft or hard.
The first thing to understand about fish keeping is that you first need to grow some bacteria. These live inside the filter and remove the waste from the fish, waste that is dissolved in the water. Without these bacteria, the fish will get sick pretty fast and probably die. Don't listen to any shop that tells you to set the tank up, let it run a day or two and come back for fish. And the bottled bacteria product they will probably recommend most likely won't work either. There are ways of growing these bacteria, but I won't throw them at you just yet. You need to get your head round the concept of growing bacteria first
Once the tank is cycled (the word for growing the bacteria) the tank is maintained by weekly water changes and cleaning to get fish poo out of the gravel if you choose to have that on the bottom. When you first get the tank, this will seem quite complicated but after the first few times you'll do it without thinking.
As for a book, to be honest, I wouldn't bother. There is a section of articles on this site and another with Q & A's. The best advice I can give you is to read those, they will give you the basics. Then ask as many questions as you need. We were all new starters once, there are no silly questions.