playing C00 French:KIA `2. d3 d5` When played C00 French:KIA 2. d3 d5, I choose to move my pawn 8. ... b4 to reply 8. Bd2. 

After that, 'the battle at early game is held' with 9. a3 bxa3 10. Nxa3 Bxa3 11. Rxa3 and at the end of game, I forced to resign in game.. 

I feel that 8. ... b4 is my first bad move because it enabled 'battle' at developing phase when I actually not ready for that. What should the right move to reply 8. Bd2 in this case??

[FEN ""]
1. e4 e6 2. d3 Nc6 3. d4 a6 4. Be2 d5 5. e5 Nge7 6. c3 Ng6 7. g3 b5 8. Bd2 b4
(8... b4 9. a3 bxa3 10. Nxa3 Bxa3 11. Rxa3 )


Best Regard, 
Ahmad
I'm answering without an engine to back me up, but I do have some knowledge of French positions and can answer on that account:

The move b5-b4 is not necessarily a bad move.  What Black always should worry about in the French is placing all his pawns on white squares and then exchanging his dark squared bishop.  It is a common theme in the Winawer variation of the French.  

Here it is Bxa3 which is the most strategically dubious move.  In the French, do not exchange this bishop unless you get something for it. Look at your remaining bishop, all closed in by the pawns.  Your bishop at f8 was stronger than the white knight at a3.

Besides, the move bxa3 is not so strong either. It opens the a-file for Ra1 and the principle for pawn exchanges is to capture towards the center, not away for it. That put aside, Bxa3 is strategically much worse as you give up the fight on the dark squares and your bishop pair for a knight in the rim.