Well, just go...

You can only go for half an hour and see what it's like, and if it's not okay, you can say you have another thing then, or have someone call you up at a certain time on your cell phone and say you gotta go or something...
Where do I sit? - uhm, I usually do this on 'comfort level' and 'go by the feeling' and by the situation, I usually let other people in and just sit to whoever I'm comfortable with and come with/after, sort of situational... where there's a place lol (?) I tend to gravitate to people I get along with anyway.. And is it a thing where you can walk around later? Then just go and move and talk to someone else?
If you like the people who persuaded you to come, chances are they'd be happy if you sit by them too? (So they won't be alone and will have you for company?? You can persuade any fun people you get along with to come too.. Maybe you even won't have to talk if they like to talk, he he.. Or you can go and mingle then, if it gets 'too much'?)
What do we talk about? So many undetermined factors. - Well, of course, that's part of the fun too..

Have some pre-determined topics: if people wear something new/unusual: 'Wow, look at you!' Or 'Wow, what's this?/where did you get this?' or commenting on their good looks is usually a good start (especially if they made an effort, and ideally if it's genuine admiration, even if of just one little detail) Also talking about the food/scenerey, especially if it's something new or exciting, or looks odd

Exciting new stuff that happened at the firm or is in plans (and is 'out' to talk about and is okay if your boss hears you too, otherwise just keep it not job-related/off-topic), maybe about any plans of theirs or yours to travel or go somewhere exotic... (if any) If you know them from before, follow up on their hobbies/interests/plans from before, especially if you two have the same interests? (or even kids/health/gym memberships in a breezy way or more in-depth if so happens and it's interesting, and if you're up for it?) If you don't hang out much with them otherwise: 'Hey, what have you been up to (these days)?' depending on how much you see them.. People love to talk about themselves anyway, he he... When you find out their interests and recent undertakings and any 'cross-over areas' that both you and they share and find interesting, it's much easier to talk about things... Have a few 'easy answers' for any questions you might get too.. (especially if some of them might be in the 'akhem' area.. Like, if they are pushy and want to know too much, prepare some answers.. If it's not like that, just joke around or say something but say nothing really.. be mysterious a bit..)
What are the worst things that can happen at such events and what can you do about that? (We can help you brainstorm too if needed..) Depends on people there, are they male/female, big or small firm, what kind of things happen, lol..
If you're one of the younger people there, you can just sit and listen to other people talk, and smile and be pretty? he he..
It's easier to sit at home and watch t.v. but I know that's not good for me.. - He he, yup, got this one right.. If you want to minimize any bad effects, stay away from sweets and alcohol as much as possible, or only consume in small/moderate amounts...
You can also give yourself a 'mission' eg. talk to xy amount of people, or have 3 or 5 interesting conversations (?) Or observe people who are 'good at this' how do they handle it? You can learn a lot!! - Or to observe waste management at the event and report to Feathers

- do they recycle? Is it eco? Is there excessive waste? Could there be less? - I've been at events where I've gone strictly for eco purposes and dreaded going there and they turned out FUN!
Soo, hope you have fun!! (And if not, you got to show off yourself in your pretty gear and practise anti-SA procedures?? Just going somewhere is brave sometimes, and kudos for going!!)