* It's not too early to start hydrating. Go get a liter bottle of water right now, and drink as much of it as you can. Finish it later, and then get another one. The easiest fasts begin very well hydrated.
* If you'd like to know how tisha b'av interacts with Shabbat, this blog post sums up most of the details of what is done with regards to havdalah and so on. Technically speaking, this is the last time in about 13 years that tisha b'av falls on a Sunday (though the fast does get pushed from Shabbat to Sunday sometimes, the next time being in about 4 years), so these rules don't come up that often.