fitting question- help!!

I am going for my first fitting this week and I know you are supposed to bring everything with you -shoes, undergarments, etc....my question is this, what do they do at first fitting and when do they usually schedule second fitting? I kind of feel silly for not knowing. I've lost a good 6 pounds in the last month and am continuing to lose. My dress BARELY fit when I went to try it on a few months ago (and I am already rather small--they are just cut very small!!) so I'm hopeful it will now but am also thinking I may lose more weight and am wondering if I go for my fitting now will I still have time if I lose more weight to get it just right? I'm getting married on October 10. Also do you go for three fittings and when is the last time they'll make changes? I'm so confused...this is getting crazy!!!