What are the chances of getting a site on the weekend, being first come/first serve? Is there a station at the front to purchase the site? Or do I have to find my own site?

Get there really early: 7am on holidays or 10am on normal weekends. You find your own site when it becomes free. You can reserve before people leave their site by placing a tent or something there and putting your reservation paper on the clipboard outside the site you are waiting on.

Chance? If you show up at ~10–1100, and watch for people decamping, you should have some luck. At the nexus of the crossroads there is a bulletin board with site maker envelopes, and the grounds rules. You mark your site with the tear-off from the envelope, and pay your due based on how long you intend to stay. You find your own site; there are ~20 sites, IIRC.

Be there about 10 am to get a site

Thanks for the help. How early is "really early"? Checkout is at 12 correct? And how likely is it somebody will be checking out on a Saturday?