Qualifications
It's not rocket surgery, but when your job affects people's ability to see, they'll want to know that you're qualified. At the very least, you'll need a high school diploma (or GED) since you'll need some math knowledge and basic optic science. A couple business classes, especially those dealing with management, can't hurt, either.
Then, you'll need to get an opticianry certificate or degree. Depending on your program, this will take another one to two years after high school. You could also opt to do a two-year apprenticeship, which really benefits the kinetic learners out there. And, naturally, you'll need a license, which, depending on the state, may require you to pass a qualifying exam (source).
Beyond those more paper-based qualifications, you'll need to cultivate some more, shall we say, intangible skills.
We're talking nimble fingers to make small adjustments to a customer's glasses as well as the people and salesmanship skills to make small adjustments to both a customer's attitude and wallet (without them necessarily knowing it). You'll have to be pretty organized, too, since you'll be helping keep the records.