Firstly, find some current pictures of Dragons - the older and more mythical..the better. The best type of dragons are those of a traditional scaly dragon, with wings. Inspiration can be found by doing a search for "Welsh Dragons" via Yahoo Images.
For the first Dragon Sketch, roughly place the head, body and legs. The flow of the spine and wings are important but basically just figure out the composition first.
Develop the dragon by adding a "lizard look", change the head to a more reptilian style and remember the eye shape is very important.
Refine the drawing by adding muscles in the joints and limbs and linking the anatomy. At this point its useful to go back and reference your Dragon Sources.
Shading and cross-hatching are important in adding depth to a sketch. It's easy to over-work a drawing, but if detail such as claws and scales are added it will improve the Dragon.
At some point in time you may want to colour your Dragon. The usual colour is muddy greens, but I like to with reds or dark brown where the Dragon could have been scorched by it's own flames!