It's been a while since I've been to a larger-scale premiere or party but it all depends on the individual actors, what the overall mood was like at the screening itself, etc. Just play it by ear, but here's some things that might help.
Before the event:
If you cut your hair before-hand, do it at least a week in advance in case it gets screwed up.
If you are buying new shoes, break them in ahead of time so you don't get blisters and look uncomfortable. Wear real shoes, not tennies.
Dress nice and nowadays you don't really need a full-on tux (with the exception of awards ceremonies), but business casual or something kind of stylish w/out being showy or pretty much just whatever you are comfortable in unless it's a t-shirt and a baseball cap. Don't wear that (the cast/crew or director might though if attending but that's them).
Bring cough drops.
At the screening:
Follow the staff and usher seating instructions if given. Don't wander around any reserved/roped off sections (unless you are asked to be part of "background" for any photo-ops).
(edit- my husband mentioned this) Try to ignore the cameras if you can i.e. don't look straight into them.
There may be a couple of "fashionably" late arrivals (who may or may not be in the film) for showboating reasons. Humor them if you want by applauding but only if someone else starts it.
There will probably be an introductory speech before the film starts. If you are lucky it will be only one speaker and it will be short.
If the San Jose Film Commission happens to be there and you happen to be seated by them, you may politely "shhhhh...." them as needed when the film is running or quietly remind them to put away their cell phones. They will glare at you, but that's okay. They are more likely to show up at Bay Area screenings than SoCal but you never know.
The stars have to pee like anyone else- some theaters might have private areas but if not and you see anyone (a/b list whatever) in the restroom it may be tempting to start up a conversation. Resist that temptation (unless they talk to you first) and try not to stare.
At the party:
If you feel better drinking, do so but in moderation (already mentioned).
If an actor wants attention they will make it obvious, if not, leave them be.
There may be a lot of schmooze. People may approach you to figure out who you are. If they seem sales-y or in "networking" mode you can head them off immediately by saying "not in the industry". But you might want to talk to them anyway if they stick around after that.
If you happen to end up in a conversation with an actor and say something stupid, don't worry about it and just move on (conversation-wise) instead of trying to backtrack too much. It doesn't really matter and they are used to it anyway. They can say dumb stuff too.
In general just try to be yourself. You're not competing with anyone and at then end of the day all that really matters is that you had a good time.