A fresh pair of skivvies is the ideal start to any kit. Now that you're older, you don't need your name on the tag of every pair (this should probably have stopped in like middle school at the latest). You can also afford to up your game from the boxers you wore in high school and college to some proper underwear that isn't made from 100% struggle cotton. Just be careful, you don't want people accidentally thinking your name is Calvin Klein.
Always wear clean underwear.