We all know that light doesn't radiate without a source. Lights in your map should have a place of origin. Thankfully, there's a collection of models available to use as light sources. Let's start with a simple point light source.
This is a concrete hallway with a simple point light in the middle of it. The light comes from nowhere, and the overall effect is boring and unrealistic. Let's put some light fixtures in the hallway to add some realism.
Add a prop_static entity using the light_cagelight02_on.mdl model to the map, and align it to the wall, as shown in the following image. Make sure to choose the correct skin entity for the light, as some light models can have an on skin and an off skin. Move the light entity a few units out in front of the model and reduce the brightness parameter to 80.
Give the map a quick test-compile to check the results. If the lighting on the model is looking a bit off, set the disable shadows property in the prop_static...