In myth the dragon symbolised a myriad of terrors real and imagined, perhaps evolved from the real wild beasts which roamed the ancient forests. For modern storytellers, the dragon is often the majestic hero of the story. When drawing dragons, its worth remembering that no matter how many great picures you have seen, they are all imaginary - and this gives you complete creative freedom.
So where do you start? First, decide what sort of dragon you want. What sort of environment has shaped your dragon? A mountainous desert or a green forest? If it is a mediaeval fire-breather you will might to consider historical sources, or for a fantasy creature, you can let your imagination run wild. What are the key features? Is it going to be a slender, lizardlike dragon on gossamer wings, or a mighty beast with the chest and arching neck of a war horse? Sketch out some rough ideas.
The key to a convincing dragon is anatomy that flows together and makes some kind of sense, then using real animal anatomy as a reference. Does your dragon have wings, and if so, are they functional or purely decorative? Consider the skin - leathery, fine reptilian scales or crocodile rough; camoflage greens and browns, or bird-of-paradise brights. Take a look at your sketch and decide what references you need, then hit the search-engines for reference photos. Remember to learn from, not copy, the picures - you don't want to breach copyright. Reptiles, birds, horses and dinosaurs can all be great sources of material.
To look realistic, you need the dragon to be lit by a single primary light source - be aware that when drawing from a range of reference sources, you might be looking at scales lit from one direction and wings lit from another!Identify the location of your light source (whether it is the sun overhead or through a cave or window, or a fire, or even the moon) and remember that light travels in straight lines. This should help you locate your shadows. Also remember that light bounces - so there might be a hint of light in shadowed areas, bounced from nearby walls or the ground. If in doubt, grab some plastic dinosaurs and a flashlight sun!
Fires and fire-breath creates a whole new problem. Please, no experimenting with flamethrowers! Note in the drawing above how the light from the flame bounces off the face and body. I've used a tonal drawing style to heighten this effect, with a dark background to give contrast to the flame and smoke.