What tends to be at issue
Unwanted comments, messages, images, or physical contact, and conduct tied to a job decision where something was offered or threatened. A pattern that changed what it was like to do the job, even where no single incident seems large on its own. Conduct by a supervisor, a coworker, a client, a vendor, or someone with influence over assignments, and conduct that continued or worsened after it was reported, which is often the part that becomes the clearer problem. This happens to people at every level and in every kind of workplace. You do not have to be certain it qualifies before asking, and nothing about it is your fault.
What helps
Save messages, emails, photographs, and any report you made, and do it before access to a work account ends, since that access is frequently cut off on the day someone leaves. Keep copies somewhere personal rather than on a company device. Write down dates, what was said, and who else was present, while the sequence is still clear. Keep the handbook and the reporting policy. Note any change in your schedule, assignments, reviews, or treatment after you raised it, because retaliation is its own issue and is often easier to document than the conduct underneath it.
The paths, and the pace
There is usually an internal reporting route, and whether it was used affects what happens later. Beyond that, certain claims go first to a federal or state agency depending on the claim and the employer, while others can be brought directly. Some employment agreements route disputes to arbitration, though the rules on that have changed for these claims in particular. State laws differ in what conduct they cover, who counts as an employer, and who is protected, so location matters. There are filing windows to watch, and the one tied to an agency complaint is usually shorter than people expect, so it is worth confirming early. A first conversation is confidential and commits you to nothing.