For our Junior chromatic homotopy theory seminar, I gave two lectures introducing stacks. The first focused on the basic definitions, while the goal of the second was to introduce quasi-coherent sheaves on a stack. You can find the notes below. I have tried to keep the preliminaries to a minimum, although for the second lecture some familiarity with schemes is very helpful.
For both lectures, I have included a number of references that I think are very good for further study. Although my notes are heavily based on these sources, any mistakes in them are my own.