I've been buying female clothing since my early teens. I ordered some school uniform like clothing when I was still in school so I could look like the other girls, I've worn teeny fashion, club wear etc etc right up to now. I certainly wouldn't advise teeny fashion or skimpy clubwear for someone your age!
I think wardrobe essentials depend massively on what your style will evolve into. As Marion says everyone should own a little black dress in my view but for a whole wardrobe you want leggings, maxi skirts, skinny jeans, a variety of tops, skirts. Jewelry, makeup, perfume, likely a wig unless you have grown your hair out. ankle boots are very versatile and easier to learn to walk in than strappy sandals. Thrifting is an excellent way to build your first wardobe. I have a lot of good clothes doing that.Look out for clothing bundles that people are selling. They are sometimes hit and miss but it helps you get an idea what suits you and what sizes to go for.
Size wise you won't really know. You will be whatever your genetics will allow. There is no way to know really. I would not worry about it just yet though as body shape changes take years on HRT (though breast growth can be quick initially) experiment with styles now and as your body changes your style will too. I would strongly recommend keeping a log of your main measurements bust, underbust, natural waist, high hip and hip. This will show how you are changing on HRT and is a very useful reference guide to your transition. I have a log going back to 2015 though i was stop starting HRT back then with big gaps in between.