Light is measured in lumens, and visually demanding tasks require at least 2,500 lumens in the room. You'll want to pay attention to two kinds of lighting. Ambient lighting covers the entire area, while task lighting is directed to a specific area. How much you'll need depends on the task, but generally, the more visually demanding the task, the more lighting you'll need. You'll also need to decide whether you want incandescent or fluorescent lighting in the office.
Fluorescent light is more energy efficient than incandescent light, and the bulbs last longer.
Fluorescent fixtures are also non-directional, so they tend to flood the area more evenly. Incandescent light, on the other hand, is warmer, and can be directed and controlled more easily. Once you've determined where your office will be, the next step is planning the layout to make the most efficient use of space. Measure the space and draw a scale sketch on a piece of graph paper. Write in all dimensions, including the locations of electrical switches and outlets, heating ducts, any plumbing fixtures, doors, windows, etc.