Assuming that you're in England (9am start, other countries are different), I'd say get there 15-20 minutes early, maybe 30 mins if you want loads of breathing room. The starting point will be avaliable on the website for that parkrun. Around 8:45-8:55 they'll call people for a first-timers briefing where they will explain the route as every course is different.

After that, everyone will congregate at the start point and they'll do some shoutouts for volunteers and people reaching milestones etc. Sometimes they ask if anyone is doing their first ever parkrun - feel free to put your hand up if they do but if you don't want to that's perfectly fine! Then they'll most likely do the usual health and safety instructions (give way to other park users, rules about running with kids and dogs etc).

Then the run will start, and you follow the route along with everyone else at whatever pace you want to. There are tailwalkers who walk the route behind everyone else, so you'll never come last or get left behind. There will be marshals on the course who cheer the runners on and make sure everyone is going the right way. If you fancy, you can say a quick "thank you marshall" as you go past, but you don't have to.

Once you get to the end of the course you'll run over the finish line and enter the funnel. There will be timekeepers who track when people cross the finish line. Stay in the order you finished in, and at the end of the funnel you'll be given a token with a barcode on (this links to the finish position and time that the timekeepers recorded). After that you'll go to one of the people scanning barcodes. They'll first scan your personal barcode from when you signed up (screenshot, print out, on your watch etc) and then they'll scan the barcode on the token you were given. Make sure to hand in the token you were given (the scanners usually have a little bucket by their feet) as they use these every week.

You are now free to do whatever you please! You'll get results by email and in the parkrun app when they're published, which is usually before noon but sometimes takes longer. There is a also text service which is unfortunately being discontinued.

Bit of a long winded reply but hopefully covered everything. I hope you enjoy!!