When you're explaining your job, you usually give two pieces of information: 1) What your job title is ("teacher", "engineer", "waiter"), and 2) What kind of company or organization you work for ("an elementary school", "a company that manufactures computer chips", "an Italian restaurant".
To explain your job completely, you say "I'm a ___ at a ___":
I'm a science teacher at an elementary school.
I'm an engineer at a company that manufactures computer chips.
I'm a waiter at an Italian restaurant.