First wedding dress fitting?

I am going to my first fitting for wedding dresses. What should i wear. (bra, underwear,ect.) I do have large breast,(40F) and I don't think they would carry my size bra. And I don't have a strapless bra, and the wedding dresses I want to try on will be strapless. (not all of them, but most) Can anyone tell me what Ishould wear and expect to happen at my appointment.