Pace yourself, pace yourself, pace yourself!!!!
That is complete key. Don't try to chase the group, do what you can do. When I did my first climbing experience I was hurting all over. But I was thankful to have a triple, before I knew it I was going about 6 mph (talk about max heart rate!) and I was cranking along at the easiest gear I could. But I did it and I just paced my way through the whole thing. And relax. Don't put a death grip on your bar, also try to keep on the upper parts of your bar to try to keep your self up right and relaxed. Oh BTW
HAVE FUN!