You tell what field or industry you work in by saying that you are "in" that field or industry:

I'm in accounting.

Here are some examples of different fields that you can be "in":

• education (teachers, school principals)
• entertainment (actors, musicians, directors)
• software (programmers, testers, etc.)
• marketing (marketers for any company)
• sales (salespeople)
• hospitality (working in a hotel or on a cruise)
• accounting (accountants)