Weak squares and pieces activity

Hello everyone,

today I played a interesting blitz game(5 minutes) against a Fide Master on ICC.

at move 16. he made a blunder with Bxa6?. He lost the exchange, but the game is not so easy for me

and at move 18.. I didn't know what to do

is clear that we play to improve the pieces, but what piece I must improve now?

Nb6 is blocked, Ra1 is good, Bg7 is good, 

maybe the knight on f6 or the queen?

I thought that black has a lot of weak squares in the dark squares, his bishop is pinned, so dark squares!

and I played c4!