what do you do for lighting? well most maps contain a light_environment entity, and this is basically your "sun" it will light up your map in the most realistic style possible, there are 3 options in light_env:

Pitch Yaw Roll ( y z x )
Pitch
Brightness

Now at the right top of your window there is a black circle with a white line in it, this is the pitch yaw roll, you can set what angle you want your "sun" to come from, I often set mine to 270. Play around with the setting on a test map to see what effects you can create.

For the pitch, I often use -45, everyone has there recommendations, but play around with it and see what you get.

Brightness is the general colour of the light, by default its a horrid yellow, which doesn't look too good in maps, it looks, well yellow, if you want a daylight map choose the lightish grey colour, and it will be more realistic, if you choose orange for instance, the shadows on your map will have an orangy step effect on, which doesn't look nice, you may have to play around with your compiling on rad, to use exotic colours.

Now thats all light_environment, there are various other light entities to, another important one being "light" this is just your basic light, you can angle this as well, and change its brightness and have various effects like flickering, To choose a custom effect, select appearance and choose an effect. Lighting is an important element on maps, and you should be aware of how much of an area one light covers, you dont want thousands of lights next to each other, when one could light it all up nicely for you.

Light_spot is another entity for lighting, working in much the same way as a light, you can have inner and outer angles, which is the brightness and fading angles, it really is best to do a small room, and try some options and keep testing away to see what looks and feels good.