I'm in Japan now and it is simply awesome. I'm thinking of visiting a public bath. Is it separate for the two genders? Can I go in with a swim suit (after my mandatory shower, of course) or do I have to be completely nude? Not that I'm uncomfortable but in a completely alien part of the world, I'm acutely aware of offending cultural sensibilities. Also being a south-Asian woman, upper body size is fairly different.
The vast majority of public baths are separated by gender, so although mixed baths still exist, you're unlikely to find one by chance.
No swimsuit (or anything else) is normally allowed, as the Japanese consider that it would spoil the water (not even your hair may touch the water, in particular). Again, some special places may allow swimsuits for some reason or other, but you'd have to specifically look for them.
Other than that, the etiquette is pretty much the same as anywhere else in Japan: be discreet. No running, no shouting, etc. Just mimic the people around you and you will be fine.
Edited to add: Maybe this bears emphasis: there is no special rule for foreign women (or men). Even though there is the usual tolerance for some amount of "un-Japanese" behaviour, it doesn't go so far as to allow you to wear your swimsuit. You can consider it akin to wearing shoes inside houses or shrines/temples.