Dress shopping nerves

Hello! I have my first dress shopping appointment scheduled (yay!) but as someone who’s never shopped for any kind of formal wear, I’m wondering what I’m supposed to wear to the appointments and just what to expect overall?

Do you wear a bra when trying on dresses? I’m assuming the store associate will be in the fitting room with me helping me put the dress on, right? I’m just a little nervous, I have a rather large chest (DD) but a small frame (size 6-8) and I feel uncomfortable just having my breasts out, I’ve nursed two children and my girls hang low (iykyk 😅). It’s also very hard to find a properly fitting strapless bra for the same reason, or dresses that fit my torso and bust at the same time. Any advice is welcome!!