STYLETHREAD -- LET'S TALK SHOP!

Okay, this may be a really stupid question, but I've never gone to a tailor or had alterations made before.

I need to get a skirt of mine taken in. I know it's a simple sewing fix, but since the skirt has some pleats in it, I don't want to risk screwing it up by doing it myself. What do I need to do when I bring it to get alterations done? I mean, should I mark it ahead of time how much I need to have taken in, or will I be able to try it on so the tailor can mark it him/herself? Can I go in there assuming that they'll have a place for me to change and try it on for them? I'm just worried that if I bring it in somewhere where they offer alterations, and I ask where to change, they'll look at me like I'm out of my mind.

It's happened before when I tried to go to a cobbler to get what I thought would be an easy shoe fix and a jeweler when I brought in what I thought would be an easy ring fix and question. Each time I think they'll help me more than they do and I end up looking crazy...which is why I'm afraid to go to craftsmen sometimes