Choose your location
Look for a blank, white, or off-white wall without any patterns, holes, or scuff marks.
Get the lighting right
This can be the trickiest part. Try to take your photo during the daytime, as the sun is your most effective (and cheapest) light source. Ideally, your white wall would be near a north-facing window, or one that lets in indirect, natural light. If you need to diffuse the sunlight, hang a sheer curtain or a white bed sheet over the window.
The goal is to avoid harsh contrasts and shadows caused by hard lighting, so if you're seeing shadows on the wall, set a lamp on the floor behind you to illuminate it. This should eliminate the shadows.