Caro Kann without a plan

This is my first attempt to post a game. I hope I got the mechanics right.  I'm trying to learn the Caro Kann. I was sort of thrown off by my opponent's 2.f4, but just sort of went with the opening for about 12 moves. I think the position was pretty equal until my blunder at move 15.  But I really didn't have a plan or any good ideas for what to do with it.  Thanks for any feedback or advice on strategy.

I don't know anything about the Caro-Kann (still less chess strategy as a whole), but there are a couple of moves I notice could have been better:

3... dxe4 seemed a bit premature. Nf6 might have been better, adding another attacker to e5 whereas dxe5 allowed white to capture the pawn without consequence.

5... Nxe4 I agree with you there, not that Nxd2 would have been better, but Bg4 was worth consideration (I think), attacking the queen.

12... e5 I don't know why you didn't play Nc5 instead. Again, you could be attacking the queen. If he had replied with 13. fxe6, you could play 13... Nxe5, scoring yourself a queen for a knight and a pawn (assuming he didn't decide to just take your knight, but that would have lost him even more material anyway),

14... O-O? I agree with your castling long idea, but you would have had to move your queen first. 14... Qc7 would have been the way to go I reckon.

15... Ng4?? You pretty much summed it up here. Though I don't think 15... e4 16. dxe4 Ng4 would have been any better since it still gives your opponent a free pawn and why did you want your knight on g4 anyway? 15... b5 makes more sense.

17... a4 Huh?

25... Rd2+?? You could have played Qxh6 instead

That's all I can think of for now.

I agree with the comment; play longer time controls. Try to forget about the name of the opening and develop your own decision tree to pick the best move each time. 

Thanks so much for  your comments and suggestions, IM pfren (and others).