What should you wear when you run?

Depends on how cold it is. A lot of races allow male runners to go shirtless, but I wouldn't recommend that at any temperature below 50 degrees. If you're racing in 40-50 degree weather, a t-shirt, long-sleeve shirt, or singlet would be fine, or whatever you can tolerate. The key is, you want to be as chilly as possible without being so cold that you're shivering. Keep in mind that you're going to warm up the further and further you get into your run, and you don't want a lot of extra sweaty clothes weighing you down. Runner's World has a clothing "calculator" on their website if you need more information.