First of all, the generic answer would be to practice.
But, some more more specific advice would be to consider drawing in long quick light strokes. Do not be afraid to "mess up" a line. Go over it in the same direction lightly to form the line you wish to create until you have the basic outline of the image. Then, using an eraser of your choice ( I use kneaded erasers, no-dust erasers, and pencil erasers) erase away the strokes that do not make the line you want.
One of the best ways to do this technique is to tape your page using artists tape, or drafting dots, to the wall. Or you could use an easel or prop a drawing board up on a chair. This allows you to stand back from the page with your arm more outstretched for those longer strokes and keeps your hand off the page so you can reduce the shaking.
A great way to start any drawing is to reduce your nervousness or stress about what you want to do. Approach the picture with a feeling of calmness. I like to use a scribble page: that is I have a large piece of paper which I scribble madly on and doodle on before beginning a piece so that I feel relaxed and can draw with a more clear view of my idea.
My final piece of advice: When drawing make sure to take a break and walk away from your work to study it from afar occasionally. This will give you a new perspective of the whole rather than the minute detail.
Long-winded but I hope I helped :)