I’ve been playing fiddle for a little over a decade, casually, and I’m curious about trying a few traditional music sessions (I’m spending Sept. - Jan. studying in the British Isles). From what I’ve heard and read, it seems that repertoire is the main hurdle to overcome: that it’s basically playing tunes that “everyone knows” like Drowsy Maggie or Mason’s Apron. Is that accurate? What are some good places to start looking for tunes to pick up? Any other tips for a first-timer?
Replying only to hijack, I’m afraid, but I notice you’re in Oregon, and if you like (American) traditional music you might enjoy the shapenote singing being held in Eugene this coming Saturday. http://pacificnwsacredharpsingers.org/All-DaySingings.htm
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I don’t know if you’re familiar with it or not, but www.thesession.org is a very large collection of traditional session tunes and trad-oid compositions by members.
Find a local session, start attending, and get to know the folks. Don’t start playing right away. If ask they probably won’t mind if you record the sessions, they may even have a tune list to get you started. You may want to see if there’s a slow session locally to help become familiar with tunes. Or see if you can find some people to start one.
thesession.org has a discussion area and etiquette comes up quite often. They also have a large tune archive. Chiff & Fipple is nominally a whistle site but they have a string section.
If you get serious, retreats like East Durham Irish Arts can be a blast, lessons during the day and music until the wee hours of the morning, i just came back. There are a few weeklong festivals and some weekend only ones too.
It can be frustrating at first, but eventually you start have enough tunes that you will be playing more than you’ll be sitting. I’m almost to that point. 
I agree with beowulf573, Don’t just hop up there and start sawin away. Get to know the people, Show interest and they will invite you to play, at which point probably decline and ask them for some tunes for the next time the session is underway. They also may have beginners folk nights or a similar sort of session to get new players involved. seek those out