I'm trying to put a light fixture on a junction box that was designed for a ceiling fan. The box has a single black wire, a single tan colored wire, and another black and white wire that have been soldered together. I have tried hooking the lights black and white wires to the combination of wires, but it hasn't worked yet. Suggestions?

Sounds like you have a feed and a switch loop. Have you tried black to black and white to tan (which I assume was white at one time)?

How about this guide for installing a ceiling fan by wire color (http://www.ceiling-fans-n-more.com/ceiling-fan-wiring.php)?