My advice is don't overthink it beforehand. Get involved, the host(s) will more than likely put you with some people who aren't the house band for a go. In this case it's something usually simple so everyone can get through it. Just get up there and enjoy it.
You'll figure out what you need to work on in future afterwards, especially as you'll then have a feel for the musical environment.
It's always nice to meet new like-minds.
Have fun!