I want to be able to crouch anytime not just when I get into the bush so is there a button for it or something?
From what I can recall, you can't. I haven't been able to find out if this has changed since I played.
Here is a quote from Steven Masters (Lead Game Designer) I found on this Kotaku article:
Looking a little closer, it was in response to this question on the Reddit AMA.
Doesn't look like there's a way to crouch outside of vegetation.
I personally have Assassian's Creed 3, but sadly, you cannot crouch by button, but by hiding in bushes. I know because I am currently playing a stealth mission. It's hard.