One trick I like to use for the first lighting pass is to plug an ambient occlusion texture node into the ambient color of the default lambert material to get some basic fill lighting in the scene (Fig.04).
For this, I make sure my renderer is set to mental ray and then find the mib_fast_occulsion node in the mental ray > Textures section in the Hypershade. Once I create this node, I drag it into the ambient color section on the default lambert material. I can now make adjustments to the light color by changing the Bright value in the ambient occlusion node.
I next add a standard Maya directional light with Ray-traced Shadows turned on to represent sunlight coming into the room. I'll also cut some windows into the walls of my room shape using the Add Edge Loop tool to subdivide the walls a bit and then delete the faces where I want the windows to be.
The position of the windows and angle of the sunlight light are both important parts of the composition. I like to adjust both to try and find the best results. For example, widening or shortening a window to control the shape of the light hitting inside the room.
It's also worth trying a few different combinations of light angles and window positions/sizes at this stage. Even though I may already have an idea in my mind of how the light will fall on the scene, sometimes you can get some nice results with trial and error. I also experiment with the color of the key and fill lights at this stage to start to set the mood for the scene (Fig.05).
I then add some area lights to simulate light bouncing off surfaces and coming in through the windows. I place these lights near to where the sunlight is landing in the scene as I'd expect to see a lot of light bouncing around there as well as the areas around the windows. When I'm happy that I have the lights in the right positions, I move on to experiment with the color and intensity of all the lights I've added until I feel I have the right mood for the scene (Fig.06).
Finally, I create some geometry to represent blinds over the main window. I felt that it would be helpful to have some break-up of the light coming in through the window. This adds some nice shadows and is a good way to help the light describe the shapes of your geometry (Fig.07).