HR will tell you some initial information about why you are there. You may or may not be told of exactly what you're alleged to have done. They'll advise you of your rights and responsibilities regarding telling the truth, not retaliating, whether you can discuss the matter with others, and if so, with whom. They may also provide you a copy of important documents such as relevant policies (e.g., nonretaliation policy, investigation policy, EEO policy, etc.).
You will then be interviewed and the HR investigator will typically take notes. (Do not take notes yourself, or ask before you do.) It's often a one-on-one meeting but not always. If you belong to a union, your union steward will obviously be there. Sometimes HR brings in other HR employees as witnesses or co-interviewers.
At the conclusion of the interview, you'll usually be reminded about nonretaliation and other responsibilities. If HR does not describe the investigation timeline/process or give you direction regarding whether you can return to your regular job and work as normal, ask. If you have information to offer in your own defense, by all means, do so. Also, be sure to get the name and contact information of the interviewer so you can follow up if needed.