So, what exactly is lens flare? The short answer is that lens flare is the result of errant light being reflected internally among the glass elements that make up the optics of the lens. Lenses that are more optically complex tend to have a higher incidence of lens flare than more simple lenses.
Creating lens flare in your image is a relatively easy affair. Simply compose your image facing towards a light source. A small, extremely bright, directional light source such as the sun, a streetlight, spotlight, a simple indoor lamp or even a flash are ideal for creating lens flare.