This is one of your first steps down the pathway of exiting rookiehood and entering Real Copland. Let's go over some the facts and fantasies about complaints, whether to the precinct sergeant or to IA. There are several truisms here to review.
Refer to your rules and regulations for guidance. Understand the procedure and what the policy states about these irritable moments of life. If you have a union representative, benevolent group or a trusted senior officer, consult them immediately for advice and their counsel.
The vast majority of complaints are not career enders. Most of the time these blindsided complaints are simple ones. They are often categorized as a lack of communication leading to a misunderstanding or perceived rudeness in their eyes. If you've been involved in a critical incident, major use of force, ended up on the six o'clock news, or some other form of police merriment, then this was probably expected.
Don't panic or think that this person is going to get your home and car in a court-awarded lawsuit. I've never met a cop who lost his home to a bad guy. However, I know several who've lost perfectly good homes in divorces.
Settle down. If you have any idea who or what the complaint is about, start gathering reports, tickets, or notes. Was there a cover officer with you? Get your information and defense straight.