In your case it could be either ''at" or "for" • You work AT a particular place or organization: She works at the Library of Congress. • You work FOR a person, company, or organization that employs you: He works for his father. We both work for the same company. Other preps with work: You work IN a city or area: He works in Manhattan. • You work in a type of place such as a bank, shop, or factory: She works in a library. • You work in an industry or a type of job, for example education, advertising, or IT: He works in advertising. • You work AS a waiter, cashier, accountant etc: She works as a cook.

If you are trying to name the place where you work, the correct construction would be, "I work at XXXXX." for example: I work at the store. I work at the factory. I work at Microsoft. If you are trying to give the geographical location of the place where you work, you could use the construction, "I work by XXXXX" (or even better, "I work near XXXXX"). For example: I work by the Italian restaurant. I work by the highway. I work by that large office building downtown. To my thinking, though, "I work near XXXXX" will sound more natural in most cases when referring to location (that is, rather than "by").

"I work for ......" is the most usual way to tell someone which company employs you. You can also say "I work at ......" , if you want to specify a particular location. Do NOT say "I work by..." : this is a classic error made by German speakers. Your German studies have clearly confused you - remember that the German 'bei' is not the same as the English 'by'.