This 'How To Mosaic' outlines some tips and resources to get you started designing your own mosaic.
Inspiration can come from anywhere; books, magazines, the internet, the environment and photographs (just to name a few). Unless you are great at drawing freehand from memory you will need some type of source material to reference your design to. Developing a reference library of your own is a great resource and can be made up of photos, images or patterns from the internet, pages taken from magazines. Google image search offers an amazing selection of images on virtually any subject.
The basic rule of thumb when it comes to designing your image is to keep it simple. A cartoon or an outline of your design is all that's required. There is no need to add shading or detail to the design, the detail will come from your materials and grout.