Lisa's right...white cotton.
If you have a Men's Wherehouse near you, they have pretty good service and will make sure you fit into a tuxedo shirt that's appropriate for your size and the style of your tuxedo. They have cool accessories too (cuff links, studs, bow and straight ties, suspenders, vests, cumberbunds...and silk pocket squares).
Congrats on bringing the tuxedo along. I still do, and wear it on every formal night.