Wear something casual but classy. So not jeans + t-shirt, but not a suit either. A dark colored dress shirt (maybe with some striped color, leave one or two buttons at your neck open) + dark pants with belt, just keep it simple.

If you normally don't drink, just order one beer every 1.5 hour at most. People drink alcohol for a reason, because many people are slightly nervous at such a party. One beer (or wine, depending on how classy the restaurant is) at the start will help you feel at easy.

Don't make forced jokes. Just laugh at theirs, even if they aren't that funny. You can however think of a few things / questions to talk about to your (table) neighbors beforehand. Innocent stuff: talk about work, their taste of music, what they think of the food, what movies they've watched recently.

You are new, nobody will be expecting you to be the center of the party, but try to socialize with at least a bunch of people.