Happy Faces on Everything is all about helping designers improve their illustration skills. This includes tutorials, articles, a strong online community and a catalog of courses that 20,000+ people already swear by.
I discovered HFOE and immediately fell in love with how each tutorial just gets straight to the point. Hayden really focuses on making sure you're learning as he goes along, too. I'd strongly recommend downloading a few of the lessons and giving them a try!
Super simple tutorials that help you add professional illustrations to your design
You could try Lynda or YouTube tutorials instead, but in my honest opinion they don't compare