Most Helpful Girl
That's what dressing rooms are for.
It's really not that hard. I personally never have a problem with finding the right size pants or what not. It also depends on if the brand goes by European or American sizes. A size 0 in the US is a size 4 in Europe.
A little trick that my dad taught me: Take your arm and put your hand into a fist, so it's from your elbow to the end of your fist, and put it into the part of the pants that will rest on your waist. I hope I explained that okay. But using that little trick really helps me.