1st bra fitting--any tips?

I've never thought I needed a bra before, and I've never even owned one, but I'm someone with sort of larger and since they're sagging I've decided to get fitted. Is this a good idea? I want the most accurate fitting possible, where can I go, and what is it like? What should I wear? I don't have a problem removing my top/bra, are these fittings proven to be more effective? Also, where in the store do the fittings take place and can the sales person help me with my bra fit/putting on my bra? Please help thanks