You can always Google whatever you're looking for ("latex underline" or "latex Greek letters" or "latex fractions" or whatever) and you'll always end up with good links. After a while you'll start to remember things.
There's a stackexchange site where the experts discuss the fine points, of which there are many.
And this is a great online editor where you can try out complex expressions to see if they're right.
Also you can look around the forum for posts containing markup and Quote them to see how they're done.