Savoy, it’s not such a dumb Q, but it’s tough to answer in a forum. Some players think (and have said to me) that, because the first chord was Emaj, then the song’s key is Emaj. It doesn’t work like that. Question is, how much use to you is it knowing what the key is? If you’re told it’s Cmaj, does that mean anything to you? If not; don’t worry, it doesn’t mean you’re crap, but music is a bit like maths. “6×3 = 18” doesn’t suddenly tell you all about maths and CEG = Cmaj doesn’t suddenly explain keys.
FWIW, I’m rubbish at reading and interpreting music, so I’m not trying to patronize. What I’m saying is, you won’t get a one-para easy answer. If you want, or have to, understand music theory then you’ll need a book or a DVD or a website and some quality time with the instrument. Sorry if it disappoints. You could always try ‘winging it’, I know plenty of great players that just let the fingers fall in the right place. In their own way, though, they still had to work at it. If a forum bloke does explain all in one post (like I say, I’m no expert) then (s)he is a god.