BakaAoi chan, dou itashimashite!
XL stands for extra large, right? Then I'm afraid it will be difficult for you to find shirts that will fit you here.
We usually have only sizes 7, 9, and 11, which is 4, 6, and 8 in American sizing. There is a "big size section" which handles sizes 13-17 (in American sizing, 10-14)in almost every shopping mall/store here, but the design sucks. The big sized clothes are generally designed for middle aged and older women and you are only 16. However, imported jeans must have bigger sizes and you know, we girls can look cute in men's shirts too!
Oh, just remembered, we also have sizing S, M, and L for both men and women, but I assume, our L is American M or maybe S. So I recommend you to bring as much clothes/shoes as you can from home or probably your parents can send them to you anytime.
Anyways, BakaAoi chan, I hope you will make it to Japan some day and have a great time, ganbatte ne!