Maybe you want a "Sun," or do you want like a light fixture that emits light?
There are textures that will automatically emit light. Apply one of these textures to a brush face and you are done.
In some instances you may want light where no obvious source is available that can hold a lit texture. In this case you would create a normal brush them convert it to a light entity. A light entity is a small diamond shaped item (might be purplish in color) that will emit light according to your settings. In BSP Jitspoe has put a good explanation of entities and their variables if you click the little "?" when on the entity window with an entity selected.
Lastly, you may want a "Sun" to provide your light. A sun is just like a...well....Sun. It emits light from the sky at definable angles and intensities and color and with definable ambient light...all kinds of things you can do there. If you bring up your WorldSpawn window (the one where you set teams, sky, message, etc..., and select the "?", Jitspoe has been kind enough to include very nice descriptions and examples for Sun values, along with all kinds of other WorldSpawn related tags.
And, what would be an explanation of anything without external link support?
ArghRad: the Sun creating tool of DP:
Awesome explanations on this page.
http://arghrad.planetquake.gamespy.com/RUST!!!: The good old place for all your map editing needs. Great writeups on most everything concerning Q2 mapping...AND MORE!!!
http://www.gamedesign.net/node/27Happy mapping...don't accept mediocrity...Take the time to get it right and make something that will blow us away!!!