The letter will tell you to bring shorts and t-shirts, but there is no need because you are given clothes as soon as you arrive at Bancroft. Your squad leader will give you a bag full of clothes (around 5 t-shirts, 2 pairs of shorts, and a sweatsuit) and will tell you when to wear what. You won't have the opportunity to wear any of your own clothes until you leave the yard for the airport, so my suggestion is to simply not bring any. The only things you really need to pack are socks, underwear, a couple pairs of running shoes, a towel, and sheets. A small blanket may also be a good idea, but it gets pretty hot so you might not need it.