How to tell which octave to play in on the piano?

I am new to the piano and I want to learn all of the basics before i actually start to play. I have been looking for the answer to this question but I can't seem to find it anywhere. Where does the sheet music tell you what octave to play in? I can read notes I just don't know what octave I am supposed to be playing in. Any help would be greatly appreciated. Thanks in advance.