What plan should black have followed in this game? I played as black in the following game, and with the help of StockFish I've discovered a few mistakes, one of which was a very serious tactical blunder. However, I'm having trouble figuring out where I went wrong strategically.

[FEN ""]
[Event "?"]
[Site "?"]
[Date "2014.03.04"]
[Round "?"]
[White "Stockfish DD (50.0%)"]
[Black "Ralph"]
[Result "1-0"]
[StartPly "16"]
[StartFlipped "1"]

1.d4 e6 2.Nf3 Nf6 3.c4 d5 4.Nc3 Bb4 5.a3 Bxc3+ 6.bxc3 O-O 7.e3 Nbd7 8.Bd3 
c5 9.Bb2 dxc4 10.Bxc4 Nb6 11.Be2 Nbd7 12.c4 a6 13.a4 Ne4 14.O-O b6 15.Qc2 
Ndf6 16.Ne5 Bb7 17.Rfd1 Qc7 18.Bd3 Nd6 19.a5 cxd4 20.axb6 Qc5 21.Bxd4 Qc8 
22.c5 Nd7 23.Bxh7+ Kh8 24.cxd6 Qxc2 25.Bxc2 Nxe5 26.Bxe5 f6 27.Bg3 Kg8 28.
Bb3 a5 29.d7 Ra6 30.Bc7 1-0


Here are some of the specific mistakes that I've spotted, which you might like to add to or elaborate on.


From what I can tell, 12...a6 was a mistake because of 13.a4, leaving me with a weak b pawn and a cramped Q-side.
StockFish pointed out that 15...Ndf6 was a mistake. I'm guessing this is because it allows white to place a knight on e5?
StockFish showed me that 19...cxd4 was a massive blunder resulting in the loss of two pawns and two passed pawns for white.


My original plan, rudimentary as it was, was to aim Queen and bishop at white's K-side down the a8-h1 diagonal, then double my rooks on the semi-open D-file, and finally launch an attack on the king using my knights. As it turned out, I failed to coordinate my pieces properly and was unable to even begin implementing this plan due to white's threats. I just wonder:


Was this the correct plan to begin with, and if so, how could I have implemented it better?
If not, what would a good plan have looked like?
Based on my play in this game, do you feel I need to "go back to school" to learn anything, and if so, what specifically?


Your advice is very much appreciated!
Plans are not usually so long range and specific. Your opponent will be trying to do things that you cannot ignore. Plans are often a collection of piece improvement ideas that work together to control important squares and put pressure on your opponents position.

Part of your plan could be developing your light square bishop on b7. This looks to help control the central light squares, and might long term put pressure on white's kingside. 

If you want to put your bishop on b7, then b7-b6 is a priority. a7-a6 does not go with this idea, as you point out. 

White's bishop on c4 does not have a lot to do and does not need to be driven off with 10...Nb6. This also gets in the way of developing your bishop to b7.

On move 10, white's bishop on b2 is not very good. You should look to controlling the d5-square to prevent white d4-d5. You should support your pawn on c5. It brings pressure on white's center, and if white takes d4xc5 his pawn structure will be weakened. Qc7, Rfd8, Rac8 look like good places for the heavy pieces to help control the central squares, and the possibility of white ending up with weak pawns on the c or d files. Qc7 and Nd7 help control e5, and might provide a chance for an e6-e5 break.

A knight posted on e4 goes well with a bishop on b7.

This plan on move 10 would need to be continually adjusted based on what white does.