My first dress fitting is coming up and I'm getting very nervous! I'm afraid my dress will be a little too snug and they won't be able to let it out at all

I have no idea what kind of undergarments to buy or what will work best for me. I just want my dress to fit comfortably and I'm worried that they won't be able to alter the dress the way I'd like. Has anyone else felt this way too or can relate to me? Am I just overthinking too much?