Emacs has extensive help facilities--so extensive, in fact, that we can only touch on them here. The most basic help features are accessed by typing C-h and then a single letter. For example, C-h k gets help on a key (it prompts you to type a key, then tells you what that key does). C-h t brings up a short Emacs tutorial. Most importantly, C-h C-h C-h gets you help on help, to tell you what's available once you have typed C-h the first time. If you know the name of an Emacs function (save-buffer, for example), but can't remember what key sequence invokes it, then use C-h w, for ``where-is'', and type in the name of the function. Or, if you want to know what a function does in detail, use C-h f, which prompts for a function name.
Remember, since Emacs does completion on function names, you
don't really have to be sure what a function is called to ask for help
on it. If you think you can guess the word it might start with, type
that and hit 
to see if it completes to anything. If not,
back up and try something else. The same goes for file names: even if
you can't remember quite what you named some file that you haven't
accessed for three months, you can guess and use completion to find
out if you're right. Get used to using completion as means of asking
questions, not just as a way of saving keystrokes.
There are other characters you can type after C-h, and each one gets you help in a different way. The ones you will use most often are C-h k, C-h w, and C-h f. Once you are more familiar with Emacs, another one to try is C-h a, which prompts you for a string and then tells you about all the functions who have that string as part of their name (the ``a'' means for ``apropos'', or ``about'').
Another source of information is the Info documentation reader. Info is too complex a subject to go into here, but if you are interested in exploring it on your own, type C-h i and read the paragraph at the top of the screen. It will tell you how get more help.