Adobe Illustrator contains a wide variety of tools and features, and has somewhat of a steep learning curve. The best way to learn Illustrator is to start at the very beginning.
Start a new workspace by going to File > New
Here, you will choose the size of your document, the number of art boards, and other options such as color mode; all of which can be revised later. Common screen sizes for mobile devices can easily be found through a quick search, as well as dimensions and guidelines for Facebook cover photos.