One simple method is to paint the walls of your room the desired color. Alternatively, you can pin and stretch fabric sheets behind your scene. Muslin is a common choice of material. Green-screen backgrounds can actually be any color, but green or blue are preferred because cameras tend to pick these colors out well.
Chroma-keying requires good lighting, a crease-free nonreflective background and a good recording setup for convincing results. Professional keys also often involve post-processing enhancements in software.
You should prefer a matte backdrop when possible, because shiny materials cause specular reflection. These reflections manifest as bright white blotches on the backdrop that make keying difficult. Matte materials also throw less colored light back into the scene. Diffuse reflection can cause backdrop-colored borders around the edges of subjects, especially when dealing with hair. This problem is especially severe when subjects are positioned close to the backdrop.
Green is a popular color choice for backdrops because it is quite distant from ordinary human skin tones. Another advantage is that green clothing is unusual, whereas blue and others are relatively common. Clothing or skin that matches the backdrop inevitably results in a poor chroma key, though results can be amusing when it is done intentionally.
The most important component of green-screening is lighting. High-powered bulbs, reflective umbrella props and flexible stands are often necessary to get a production-quality key.