Penciling is done by an artist based on the writer's script. It is the penciler's job to make the writer's vision a reality. While I cannot possibly cover all of the aspects of penciling in this website, here are a few good points to start with.

Characters

As with writing, you should put as much thought into your character sketches as possible, making them each unique and giving them a life of their own, using the character bios given by the writer.

Anatomy

If you are really serious about becoming a competent penciler, you'll need to study anatomy, if only the rudimentary aspects of it.. Here are some examples of how to draw the human head and the human form.

Head

Body

Basically, just practice as much as you can. That's the only way to get better at anything.