Before playing a move, think about what your opponents wants to do.
Bb5, what else if not taking on c6?
So no need to play a6, u just go into white's idea...
Now what instead of a6...
Avoiding pawn doubled is not easy, u dont have really good square for your Nc6, and only way to defend it is with your queen, c7 or b6.
Thats possible but u could also think that if u want to take on c6 with queen, then white will be far ahead in developement, could then open with d4, and it could turn badly for u.
So the best is to ignore, u have choice, depends on what open sicilian u like.
If u like fianchetto, g6 is the move.
The other main move is e6, to follow with Nge7 protecting the Nc6, so provoking (but not forcing) Bxc6 before, then taking with b pawn is the main capture, to strengten your centre and to open b file.
d6 is also possible, and Nf6 too (at move 3), but take care with Nf6, e5 could be an idea for white at some moment...
I came across this interesting variation of the Sicilian defense in one of my live chess games. I never seen it before and couldn't find too much on it. How should I have responded to Bb5? Here's the game: