im making this fairly small room and its pretty dark, so I need to add a light. I put one in there, and it leaves this very bright spot on the ceiling and the floor. How do I get rid of that?
Spotlights (light_spot) will shine a cone of light in a specific direction. I find that most of the time, these are superior to regular lights. https://developer.valvesoftware.com/wiki/Light_spot
You can adjust the falloff settings and brightness if you want to make lights look more subtle instead of very bright. If you want to light up your entire map consistently, I'd recommend using a skybox.
The dev textures you're using won't bounce much light (Reflectivity 10%) so the room won't be lit very well. Try a higher reflectivty dev texture or consider just making the lights brighter while you're in the dev stages (since you'll be changing the textures in detailing anyway).