The stroke window lets you adjust the parameters of your line. You can choose how thick the line is, what kind of shape appears around tight bends, and what we came for—a dashed line effect.
To create a dotted line effect, check the Dashed Line line box in the Stroke Window.
Once you've adjusted your dotted line settings, click away from the line with the Selection Tool to leave the stroke in place.
Congratulations! You've created a dotted line in Illustrator. You may also be interested in our guide on how to disable the start screen in Adobe Illustrator.