Mistake in the opening?

For my first post in the forums, I've decided to ask for some help on one of my beginner games.  I'm black in the above diagram.

I lost this game in only 12 moves, so I assume I made a major mistake somewhere.  At first I thought it was in the opening, but after looking up the sequence of moves that occurred, it was my opponent who first deviated from the standard sequence, around move 3 when he moved the Knight.  Was this a mistake that I could have punished more harshly than I did?  Also, I'm thinking that taking his Pawn with my Knight was a mistake; am I correct?

Obviously, I miscalculated on the last turn of the game, and he put me in checkmate.  Approximately how far behind was I at this point; was the game still salvageable had I not miscalculated?

Essentially, all comments are welcome.  As stated above, I've only just begun studying Chess seriously, so please point out in gaping holes in my knowledge that you find

u hung a knight

Openings are not your problem.  The ability to think to yourself and say "what did my opponents last move change?" is your problem.  7. Nd5 put a knight between your queen and your knight!  Oh noes, I would immediately play something like Ne6 to protect it.  But you failed to check what whites last move changed!

Furthermore, after 11. Qd5 "Oh noes, the f7 pawn must be defended or its checkmate in 1" I thought.  I saw that the queen moved to somewhere where f7 (right next to my king!) was threatened and quickly found out that Qxf7 was checkmate!  But you didn't, instead you played f6?? and lost.

besides hanging the knight,  i dont think the king move on move 8 was a good idea you should have played f6 this would have saved your pawn on g7 and the rook on h8.  Even if you did play that you are still pretty far behind in development and also down in material.  Also on move 11 when you finally played f6 is when you should have played Be6 to get a piece developed with tempo.

consider 2. ... c5 to prepare to teh bxc4 with b5, so you would not have to go after the center pawns so vigorously.

11...Nh6  would have worked.  8...Kf8 was a wasted move, by moving out the kingside knight and then castling you could have prolonged the game.  I think even after hanging the first knight you could have put up a fight.  

Your opening play was first class, and as other people have said not the problem at all. 

         2... pxp QGA is perfectly good opening, fzweb's comment and whole "analysis" in fact, is classic analysis by result. 3.Nc3 is sharper than many people realise because black can consider hanging on to pawn with 3...a6 4.e4 b5. Nc3 is helping black's counter-play with b5-b4. 3...Nc6 is a perfectly good move. 4.e3 is a bit limp-wristed to be honest. 4...e5 very good, black should be looking to play this if white hasn't played Nf3. 5.Bc4? poor move, white should probably try to bale out into an equal position with 5.Nf3 pxp 6 pxp Bd6 7Bxp Nf6 etc. Instead of 6...Nxp think black is close to winning with 6...Qxp. Don't think 7.Nd5 is all that impressive really, after 7...Ne6 white doesn't seem to have a lot for pawn.