between friends I assume?
just draw straws to see who plays eachother and do brackets from that.
Drawing straws would be ok... but it's always fun to actually make teams. What we did was pick captains and the captains had to build a team then.
Other than preparing teams, it's always nice if everyone pitches in to buy food and drinks for the event. Make like a $5 per person entry fee and it all goes towards food and so on. Possibly a cheap trophy???
I'm guessing it's going to be on private land correct? If it's a woodsball tourny than your fine or you could build a feild. It's up to you.
One this we did was make a set of rules for the tourny to try to prevent cheating and so on.