Hi Sarah
Welcome Honey. There are lots of posts about shopping and also stuff in the Wiki guide. Basically very few shops will have any issue with you buying female clothing, they are there to make money. Many people feel very nervous and unsure about fit and sizes etc. If you have been smart enough to ask for help already then that is the way to go. Not all shops will let you try stuff on, but many will. Some of the big stores may ask you to use the male change room if you are presenting as male. Many of the smaller ones will just let you use a stall no questions asked, just ask the staff; "Ok to try this for size?" No need to embarrass your self. No shop assistant will shout out, guy here wants to try a dress on, is that OK? Thsy would be fired on the spot (BTW I used guy just as an example and not as an insult

).
From the shops you mention I think you are in the UK? I'm in Australia. Most of the chain lingerie stores here have no problems in doing bra fittings for TGs. Again quietly explain and it really help if you have inserts with you. Hi I'm TG and wanting to be fitted for a bra, I have my 'falsies' could someone measure and fit me? No problems. When I did that, the assistant even took me around the store and showed me different styles and what not to buy. Such as it's a waste of time using a push up bra if you have nothing to push

.
Of course once you are out in the world as the female you, nothing matters. You do what you want.
Shoes, I have tried on female shoes when in drab, again no problems or questions. Yes my toenails were polished so it was pretty obvious that I was into that type of style.
Make up, I have to admit I started from scratch and experimented. I did go to a professional make up artist who gave me tips. I found then through a local TG club to whom they advertised.
Wigs, I bought my first ones on line, I now go to a wig shop to be fitted and styled. Very helpful. "I know you like the to the waist length blonde one but do you think many women in your age group have waist length blonde hair?" me "Ahhmm No" In your case being young it would be different.

As many others have and will say, presentation is confidence. Be the confident woman you are and you will have no worries. Finally if you go to particular shops and get good service go back to them. If they are small ones they will quickly get to know you, and probably sign you up for email and loyalty alerts. Let them know what sort of stuff you like and they will be emailing you as soon as stuff comes in. It's all about the money.
Hugs, and welcome
Cindy