Like are there things that I shouldn't mention or just leave out? I've never really worked with an HR department before so any insight is appreciated.
I would be careful about being totally honest. An HR representative is not your personal advocate. Their job is to do whatever is in the best interest of the company. Complaining about your boss or the company could get you into trouble, so I would tread carefully before sharing everything.
Yes. It's so important be open and honest with HR to ensure your happiness at a company, especially if you don't feel as comfortable speaking with your manager. However, make sure to keep your tone and body language professional. If you are emotional, take a few deep breaths before your meeting to collect your thoughts.
It is important not to deliberately lie to your HR department. However, there are many things that you probably shouldn't share. Remember, this is a professional environment and we are all considered adults in the workplace. Because of that, it is imperative that you maintain professionality-- as it says in other answers, don't badmouth people (A-- that's only polite, B-- that could come to bite you, C-- that makes you look like you don't work with others well), don't bring up personal details (such as your life outside of work), and don't go into a meeting with a poor attitude. If you are meeting with HR about harassment, for example, you need to include all the details regarding the harassment. If you are trying to discuss insurance/benefits, there's probably less that you need to lay out on the table, so keeping an amicable, yet guarded lid on irrelevant topics might be more beneficial.
There is a difference between HR and ER (employee relations). the HR department works with the managers on hr issues, policies and procedures. the ER is the employee advocate. So if you are issues with your manager - ER is probably your best bet.
That is a really sticky question. I mean I am usually in the column that says total honest is always the best method, however there is honest and then honest. Everyone has their own level of comfort where their personal life is concerned. I think that more and more their is pressure in the work place to insinuate the company into what is personal matters. If their is something in your personal life that has no affect at all on your job performance than there is no reason to disclose that if you do not choose to. As well as if their is something going on at the office that you know about that is not impacting the work flow or company matters that is also not something that needs to be disclosed unless you personal feel that you absolutely need that out there in the air. Just remember than once it is out there and you are the one who put it out there that genie is out and will never go back in. I hope my rambling helped.