I was very glad I was a union member- they were fantastic. I knew I was innocent, but the fact it got so far, I wasn't sure what the outcome would be, and if they had found me guilty, I would have been dismissed. I was found innocent, so all was well, but it was a very difficult time.
That's a different situation from yours, though. I have also sat in a disciplinary with a colleague who was guilty - you should be allowed to take a union rep or colleague with you, and I strongly recommend this - it's a very stressful thing to go through. Your accompanying person cannot defend you like a lawyer, but they can take notes and ask questions about points of procedure.
The disciplinary procedure would ideally be outlined in your code of conduct , or, as mine did, say something like, "we follow the ACAS guidelines", which are available online - and it's a good idea to read them anyway, as it helps give an idea of what to expect in a well-run disciplinary.
I would admit you have done wrong (hard to argue otherwise, if they have the evidence,) but claim you were stressed and will never do it again and just keep your head down. Hopefully it's the sort of thing which will be cleared after a year, but with us, it would prevent you from being promoted or applying for any internal positions within a year, if guilty but not dismissed. It also affects payrises and bonuses. So in your position, I would want to know what the outcome will be, and how long it will last. That should be made clear to you, but you should ask if not.
I tend not to speak about work at all on social media, apart from positive stuff on LinkedIn - though I am a member of a couple of closed groups, which helped me let off some steam. Even then, I tended not to mention the company name.