Hi Jessica,
as far as shopping goes, I can say from experience that it seems to get easier with time. Getting your first set of women's clothes while in boy mode can certainly be intimidating, but if you purchase your pieces in a busy store, the cashiers will often not even really notice or comment upon anything. And if you pretend you are buying something for a female in your life--girlfriend, mother, sister, best friend whose birthday is coming up soon--employees will often be willing to help you out. I did this when I was just starting out. I pretended to be embarrassed to be in the women's section and laughed sheepishly while I described what I was looking for "for my mother." I already had an idea of what I wanted in mind (as well as size), so I didn't have to seem suspiciously picky in front the girl. The girl who helped me laughed along with me and helped me find what I was looking for. It was a tad embarrassing, but I got what I needed without any hassle. And if something doesn't fit, you can return it, if the store accepts returns, by using the same story--"your mother" needs a different size, etc.
Once you get some items, you can go shopping dressed en femme if you like, and, again, generally speaking, large stores will often not give you any problems, as long as you are not already in a particularly transphobic location. You might feel more comfortable shopping en femme, as well. Of course, it's a big step to have the courage to go shopping like that, but I found that once I was wearing all women's clothing and had gone out in public a few times en femme, I not only felt a bit more confident but was stared at less. If you show you are confident, that you are just a woman like all the other women around you, people will generally leave you alone, unless you are in a transphobic atmosphere/setting. You will likely encounter some arses, but just be yourself.
Good luck with everything! And also, at 20, you still have a long way ahead. I've only begun to come out at 26, and many people transition much, much later. Don't worry too much about lost time--the future is yours.
