How do I wire a switch and light so I can add a fan later?
First, you need to ensure there is a neutral in the switch location (A requirement of NEC 2011 section 404.2 C, assuming that applies to you) -- the easiest way to do this is to run a 14/2 NM cable (standard house wire, aka the brand name "Romex") from your power source to the switch first.

Second, run a 14/3 wire from the switch to the fan. This has a white, black and red wire (as well as ground). The white is neutral. The black and red are hot wires, and generally speaking with a fan installed you'd do one of the following:


Black as switched for the light, Red as switched for the fan: If you have a "fan control switch" (commonly availale on the market) that has separate controls for light (often as a dimmer control) and fan speed.
Black as constant power, Red as switched: If you wanted a normal light switch, and then to control the fan using pull chains.


Prior to installing the fan, install the light normally, and connect either black or red to both it and the switch (see above to help you choose). You'd just cap the other (red or black) wire -- whichever you didn't use for the light -- on both ends with a wire nut. This prevent it from shorting out on anything, and is a common convention that signifies "unused wire".