“I work in the accounting department at a law firm.”
You're riding an airplane alone. You get into a conversation with a woman sitting next to you. She asks what you do. You explain.
I work in the accounting department at a law firm.
Areas of a company are called "departments". Each department usually has a name, like:
- Customer Service
- Human Resources
You can either call the department by its name or say "the ___ department":
I think you'll need to talk to the Finance department about that.
An "accountant" is someone who is responsible for tracking an organization's money: how much is coming in, how much is being spent, how much money needs to be paid to the government as taxes, and so on.
The word "accounting" describes this kind of work. You can say that an accountant "does accounting" or "is in Accounting".
When you're talking about someone's job, here's how to talk about the department or area that the person works in:
She works in sales.
I work in the facility maintenance department.
You can also use "in" when you talk about the industry that someone works in:
We work in food service.
A "firm" is a business. People often use the word "firm" when talking about a business that provides professional services:
a law firm
an accounting firm
a design firm
The phrase "law firm" is especially common. People almost always call companies that provide legal services "law firms", not "law companies" or "law businesses".