The best advice I can imagine is to keep a sketchbook, or a stack of loose paper if you prefer, and be drawing every day. The more you draw, the better you'll get. Experiment with different subject matter (what you draw) and materials (what you draw with).
Here's some examples of mine:
That's a scarecrow reporting for work in the morning, naturally.
Aquaman (copyright DC Comics) is one of my favorite superheroes for some reason.