Illustrations

I'd like to draw graphics in a similar vein to what I'm sure most people have seen here on Rob's amazing Rawkes website. I'd like to create underwater-themed graphics for a site I'm working on using the same smooth cheerful sort of style. However, my graphics skills are not that amazing.

Are there any particular things I could do to improve my own skills, or any tools/software you would recommend to help? I will also need to look for tutorials, but its very difficult when the question you are asking yourself is "how do I draw pictures"?

All advice greatly appreciated