Which Bathroom/Restroom do you use?
In order of preference;
1) Neither
2) Family/Unisex
3) Women's
How should I act in the female bathroom/restroom?
Unless you are extremely passable and *confident*, act like the invisible woman.
Is it legal to use the femal bathroom/restroom?
Maybe/Maybe not. Check local laws. Even if it is legal, the more important question is whether it's a good idea? It's not always safe.
The best thing you can do is plan ahead, and avoid the need to use the restroom altogether. If that fails, the next best thing to do is plan ahead, and know where all the unisex/family restrooms are located. Use them if at all possible.
I've used the women's restroom often without any trouble. *But*, I do my best to make sure no one else is inside when I do. If someone else comes in while I'm there, I stay in the stall until they either a) enter a stall, or b) leave. I've managed to only encounter other women a few times.