How do I wire a light fixture with one white and one black wire to a ceiling box with one white, one black, and one red wire.
Usually red is switched, black is hot, white is neutral. However, you should verify this with either a test light or a multimeter ($5).
So the easy way to do this is to connect white to white, red to black, and see what happens when you flip the switch.
However, red might be a second circuit or part of a three way switch -- there's no easy way to tell without testing it. Write down the voltages between white and ground, black and ground, and red and ground with all switches in both positions -- have your assistant operate the switches while you test them.
Finally, turn off the power at the breaker, connect white to the neutral, black to the hot or switched leg (your preference), turn the breaker on, and operate the switches to make sure everything is working.