First 18th birthday party – any drinking rules?

Loser post really. I’m 18 in 3 days, and tomorrow I’m going to my friend’s birthday party. I’ve been too depressed and tired my whole teenage years to even think of partying.
I’ve never drunk let alone gotten drunk, the only time I have was maybe some beer or champagne at family parties. My parents are non drinkers.
It’s a shame to admit but I’m SO painfully unseasoned. Are there any unspoken social rules to follow? What do I even drink? How the fuck do I go about it all?