The best tutorial I've found on Adobe Illustrator is Vector Slate's 'Vector Illustration Tutorial for Beginners'. In it, they show how to create a night-time scene step-by-step, including everything you need to know to recreate it – from the canvas size needed to the specific color codes for the sky. In illustrating the sky, you get many basic skills for Illustrator, such as how to use the pen tool, gradients, ellipse tools, and rectangle tools, all which you could then translate to your own illustrations of any kind.