Spaghetti-strap tank tops are awesome for girls.  Buy them a few sizes too big, or get the super-low kind that have extra long straps.  The long straps give enough space to get it over the RED.  Once you have it on, take the extra strap length on your shoulders and tie them up into little knots.  It's super cute but modest.

You can also get cheap button-ups basically anywhere (check store outlets, secondhand stores, and cheap department stores).

Zip-up hoodies... enough said.

In general, bigger is better, so borrowing button-down shirts from parents is another good option. Since you probably won't be going out much, your wardrobe is not that important, so bottom line: just be comfortable.