I'm just trying to think about more general tips/things to be ready for staffroom as the relationship thing has been covered really well by others.
People are really set in their ways about where they sit etc, it's true, most people aren't rude about it but there's always one or two. You can preempt this by cheerfully saying 'are we ok sitting here, we've heard lots of things about staffroom chairs?' because remember EVERY person in the room has been the student or newbie and had that same worry on their first few days. You will probably get a few anecdotes relating to that as responses. I still ask whenever I visit a new school although with slightly less care about what the answer is 😂.
Remember anyone that tries to engage in chitchat that you might find a bit uncomfortable is likely to be quite a nice person who is trying to make you as, initially at least, a visitor feel welcome. The same people talk to student teachers who also talk to supply, people on interview etc. So although you might find this tricky to navigate please try to remember their motivation is almost certainly in 'doing the right thing' and being kind. Some people just don't speak to anyone who isn't permanent. It's weird and isn't usual but it does happen, be aware of that and try not to be offended if you do come across one of these sorts.
Always take your own food, drink, cup, cutlery etc when you start and suss out how things work once you've settled, especially at the minute it's completely usual to do so (in the past I'd use any old skanky mug and raid the canteen for cutlery etc but at the moment even I'm bringing everything from home). One less thing to worry about.
Conversation-wise nobody minds students/guest/visitors giving brief answers with a smile and as others have said it's great if you can ask the other person a question as a reply. After the summer holidays the only real question is 'what do you get up to/did you get away/did you enjoy the heatwave?' sort of stuff.
As a PGCE you will get asked endlessly what your subject is, which uni, what's your commute like, that kind of thing. Unlikely to get a grilling about your personal life beyond 'got any plans for the weekend?' until you're well settled. If you want to join in general chit chat here it's usually the headlines and sport things and then people talking about children/partners/pets. And work. Try to avoid talking about kids or staff even if they make quite an impression on you as there's always some context missing when you've not long been there. Have had to have a professional conversation with student teachers occasionally when they've been spouting off about a lesson or teacher or something unnecessarily, but you don't sound like you would be doing that anyway- watch out for any fellow students who might do it though and try not to get dragged into the conversation by accident.
Having said that, if someone says 'blimey, 7C are an interesting bunch aren't they?' there's absolutely no harm in saying 'oh I'm glad it's not just me that thought that!' or something similarly generic.
You will pick up the general rhythm of a staffroom fairly quickly if you do try to use it, it's important to have breaks and I do think having friendly relationships with other staff helps students to settle in even though you are going to be very busy. If the kids see people saying hello to you in the corridor in a friendly rather than just politely that kind of subtle 'kudos' does help them see you as a more integral part of the school. I often have to encourage my science PGCEs to come to the staffroom with me or they'd be sitting working in the prep room all day every day and never get to meet anyone else.