I have a degree in graphic design so it wasn't a huge career change for me. There's lots of overlap in terms of software/skills for both design and illustration.
As for tips, I would recommend learning about light and color. Even if you're doing pen drawings, knowing how to look at an object is just as important as anything else. This was a fantastic book by James Gurney I always recommend.
James Gurney (of Dinotopia and Color and Light fame) has an excellent blog that provides tons of information on plein air painting and hand-made pochade boxes and easels.
He uses this sketchbook for most of his plein air work (watercolor, gouache and casein paints) It is small and versatile.