Obviously the first thing you need to do is figure out what to paint on the wall. When I know what I want to paint, I typically follow that up by finding different representations, photos, drawings to use as a reference, or by going to a location to take some pictures that I can use as a model. For example, if I wanted to paint a seascape I might drive around the beach area where I look for objects or scenery I want to include in my painting. Most times I am piecing together a scene to create my mural.
Once you have what you need to put together your scene, painting the background first is essential. I start at the top and work my way down. So I lay down the sky first, then the horizon, where I may have mountains, trees, water or land. After I have my background in I will place my other objects, such as people, cars, roads or whatever the subject may be. Sometimes I hand paint in these objects and sometimes I may use a projector first to cast the image on the wall, trace it out and then hand paint it. A projector is very helpful to maintain the correct proportions and to help in placement of the object.
You may be wondering what medium to use on a wall. I use a mixture of acrylic paint that you can buy in any craft or art supply store and also latex paint that you would purchase by the gallon in a paint or hardware store. Both of these mediums dry quickly and clean up easily with water. Also, acrylic paint has no fumes so working with it will not bother you during the process. I typically purchase a gallon of extra bright white paint that I mix with the acrylic colors. This will stretch your dollar, because purchasing small tubes of acrylic paint alone can add up quickly especially if you are painting a large area.
Once you have finished your mural, I suggest painting a clear coat of polyeurothane over it to protect it. Look for a coating that will not yellow with age. The clear coating allows cleanup in case of dirt or fingerprints.