Hi Sarah and
Let us know how your first WW meeting goes. You don't have to weigh-in in your undies. You'll have to take off your shoes, and its very private- its not announced. (I noticed alot of members have a "weigh-in" outfit they wear for the meetings as to keep consistent with their weigh-in, but then again I've seen people go into the restroom to change into something more lighter, or take off their jeans and stay in bike shorts). Everyone is different, LOL!
As far as the meetings go, you don't have to speak unless you want to. More than likely, you wont be the youngest there. You'll see members of all ages at the meetings.