Illustrations are super fun to create and Adobe Illustrator is an amazing tool for creating them.
There are so many different ways to create illustrations but a couple of my favourite techniques are tracing an image using a pen tablet (such as a Wacom Intuos) and/or using the Pathfinder Tool. The illustration below was created as my final project for learning Adobe Illustrator and uses both of these tools and techniques.
Tracing is pretty self explanatory. You put the image you wish to trace on one layer (and fade it out so it isn’t quite as visible) and then you trace! An important thing to remember when tracing is to play around with the fidelity. This means how smooth your pen strokes appear. Whether they are accurate to your movements or are smoothed out between points.
The Pathfinder Tool is much more complicated then tracing but can create some very precise and amazing illustrations. Using basic shapes (Circles, squares, etc.), you can create just about anything. Say you want to create an illustration of a quarter phase moon, you would take two circles and overlap them with the top one moved slightly to the left or right and then use the Minus Front Tool on the Pathfinder panel and the top shape would cut out of the bottom shape and disappear. Boom! You have a quarter phased moon!
There are so many more techniques and tools to use in Adobe Illustrator, so play around and find some tutorials!
And have fun! (Which is not hard to do with this amazing software.)