Know your breed's history, what their primary uses are, other breeds in their class..
Also know what are disqualifications in your breed, or what might be okay for your breed but is a disqualification for another breed (example: the Japanese must have a squirrel tail, this is a disqualification in every other breed... ).
Read up on poultry diseases, what preventions there are, the signs of infection, and what treatment if any.
AND WATCH THE OLDER SHOWMEN!!!!! If you have any chances at all to see more advanced 4-Hers in their showmanship classes, please do so! I know the carnival is calling, but this will give you a huge heads up! These showmen aren't always right, so make sure you remember or even write down the questions they were asked, then go find the answer from a reputable source. Ask your leader, and ask the judge after the show if they can give the correct answer to some of the questions (such as a disease question, etc.)
Look up everything chicken. As you advance in the showmanship ranks, you will eventually have to handle another exhibitor's bird, so will have to know about that breed as well!
Always fun! Learn about different color patterns, comb types, etc.
Most of all, HAVE FUN! 4H is a blast. If you aren't happy with your leader/group or how you do this year, don't be discouraged, look for a different leader and just look forward to next year. I have had some terrible leaders that made that year of 4H living hell, but once I got good leaders it was much funner and I learned so much more.
And if you haven't gotten your bird yet.. I might suggest trying a bantam Cochin instead of Large Fowl... sometimes you have to hold those birds forever and they get heavy. If you already have them, that's fine! You will want a nice calm bird, and teach them how to walk on the table with the pointer sticks. That's always interesting