Here are some things HR may want to meet with you about, and what you should do:
You complained about discrimination or harassment: HR must investigate if you complain about race, age, sex, religious, genetic information, national origin, pregnancy, disability or other illegal discrimination or harassment. If they want to talk to you about your complaint, don't refuse! If you do, then you wasted your time complaining and they'll note that you refused to cooperate. Instead, go in prepared.
Gather your evidence and witness names supporting how you were singled out compared to others in a different category (race, age, sex, etc.), how you (and any others) were harassed compared to others in a different category, and any comments made related to you and others relating to your category. Make notes and take them with you so you don't forget anything.
Don't complain about "harassment" or "hostile environment" that isn't connected to race, age, sex, etc. General harassment and bullying aren't illegal, so you aren't protected against retaliation if you report these.
After the meeting, write up a summary of what you reported. Make sure you say the words, "age discrimination," "sexual harassment," "race-based harassment," "religious discrimination" or whatever specific type of discrimination you reported. If you don't, then HR may claim later you reported a personality conflict or bullying instead of something illegal.
Discipline: If you are being disciplined or investigated relating to potential discipline, don't freak out, storm out, or yell during the meeting. You don't want to compound the situation by being insubordinate. Instead, take good notes about the accusation. If you are asked to sign a document, sign and write something next to or above your name like, "as to receipt only, rebuttal to follow." Then wait until you are calm and prepare a businesslike response with any supporting documents and submit to HR.
If you are asked questions during the meeting, be truthful. Some employees lie or don't tell the whole truth because they panic. If you lie or are perceived as lying, you can be fired for that, whether or not you did what you are being accused of.
Crime: If you are being asked about a crime you committed, don't answer. It's time to contact a criminal defense attorney. Don't sign anything admitting to a crime. If HR or Loss Prevention tries to tell you that you can save your job if you admit to a crime, they are lying.
Termination: If you are being fired, stay calm. Don't sign anything they put in front of you. Instead, ask for a copy to take home and review. You aren't thinking straight. You may be asked to sign a severance agreement giving up any legal claims you may have. They may even stick something in there saying you can't work for a competitor. Even if it's "just" a disciplinary document, don't sign it. They can't make you do anything now. You don't work for them anymore. Take good notes of what they are saying is the reason for the termination. Get copies of anything you can documenting what they are saying.
Don't run out of the office shouting, try to get your coworkers to leave with you, or cause a scene. The work world is small and you may end up working with these folks again someday. If you don't understand a contract you're being asked to sign, or think you may have claims against the company, contact an employment lawyer in your state about your rights. For more about what to do if you think you're about to be fired, read my article here.
If you need legal advice, it's best to talk to an employment lawyer in your state, but if you have general legal issues you want me to discuss publicly here, whether about discrimination, working conditions, employment contracts, medical leave, or other employment law issues, you can ask me at AOL Jobs. While I can't answer every question here, your question might be featured in one of my columns, or in our upcoming live video chat.