Halter is a conformation class. You enter the ring walking, and when you pass the judge, you trot into a line-up, and square up your horse for judgement. Basically, that's it. There should be a ring steward that will direct everyone.
Showmanship, however, is a pattern of leading, trotting, backing, turning, the works. Showmanship is very difficult, and can take months+ to train your horse properly.
Perhaps if you're curious, look up videos on youtube. Who's the organizer of the show? Call them up, ask what you signed up for, and ask for details of the class. What is required? Attire, what should your horse wear, what are you expected to do, what is the class being judged on?
There on the tips of fair fresh flowers feedeth he; How joyous his neigh,
there in the midst of sacred pollen hidden, all hidden he; how joyous his neigh