You might want to watch it at this show before you show in a liberty class. You want to choose music that fits how the horse goes.
The horse is judged on movement and how much of a show he puts on, you know flagged tail, they love the trot, especially if it is floaty. Your horse should go in both directions of the ring, use the entire ring, and trot as well as canter. You are not allowed to touch the horse once it is turned loose and when it is time to catch, the person who turned the horse loose is the only one who can touch it. Your helper can block it, and turn it, but never, never touch.
Look at your AMHA rulebook for the class rules, if you don't have one, call and ask for one, and there is an older version (I've already asked them to update it) online at the AMHA website. The liberty rules are current so you will be fine with the old version.
This is a fun class, always fun to watch.