If you want to learn stacks, its important to read Knutson's algebraic spaces first (and later Laumon and Moret-Baily's Champs Algebriques). To keep yourself motivated, also read something more concrete like Harris and Morrison's Moduli of curves and try to translate everything into the languate of stacks (e.g. as you're learning stacks work out what happens for moduli of curves). There are a lot of cool application of algebraic spaces too, like Artin's contraction theorem or the theory of Moishezon spaces, that you can learn along the way (Knutson's book mentions a bunch of applications but doesn't pursue them, mostly sticks to EGA style theorems). Another nice thing about learning about Algebraic spaces is that it teaches you to think functorially and forces you to learn about quotients and equivalence relations (and topologies, and flatness/etaleness, etc).