Most items (such as tops or pants) I managed to get my own size with some personal measuring tape; I figured from my measurements that I would be about a 36A for a bra, but before I bought one I thought I should be sure, so I went to JC Penny in male mode and asked for a sizing. The woman there was very kind and quick about it, with no real negative comment or anything and actually made me feel a lot more comfortable (I ended up being a 34A). So I ended up buying my first proper bra there (mind you, I already have worn women's jeans for several months as well as women's T-shirts under my button-downs, so I already had those sizes

).
As for actual fashion, I generally bring one of my friends along (I call him my fashion adviser) or go to the local Plato's Closet. For those of you who don't know what Plato's Closet is, it is basically a Goodwill for clothes from name brands like American Eagle, Aeropostale, Hollister, Guess, and other more upbeat teen-based clothing.
If you tend to be nervous or get in the mindset that things will turn out so much worse than they actually might, bring your phone along and have some actual conversations with your friends BEFORE you even go in; for me it helped shake off some anxiety and keep my stress levels low, even if we just talked about video games and the like. Also get in the mentality that you know what you are going for and make it a 'mission,' I ended up planning my first purchases days ahead so I could get myself prepared to go out and buy them, and remember that once you get sized, you can always buy online