If you drink all the beer on the boat make sure that you brought it on board in the first place! And I suggest you drink it after the race.
As far as becoming "crew" on a regularly campaigned racer, there are a few things you should keep in mind.
First - be reliable. If you say you're going to show up, do so. Nothing worse than expecting a full crew only to have one or more change their mind at the last minute.. esp if it was only because it might rain....
Watch, listen and learn. As you become accustomed to the routine, try to anticipate required actions. Talk to your railmate, ask questions - all the whats. whens. and whys that come to mind. Presumably the skipper is aware of your limited experience, most will be happy to teach you if you listen and learn from day one.
If this crew is the "knock a few back afterwards" variety - and few are not - bring your share or more, esp early on.
Finally, when the boat docks, DO NOT grab your gear, say thanks and leave. Stay to help put the boat away - there will be lines to coil and store, sails to fold, covers to put on, decks to hose down etc. Hang out and help with all that and you're sure to be invited back...