4H shows are different from state to state. First you should talk to your poultry project superintendant. He/She will be able to help answer some of your questions. Second you should try to get a copy of the show list, from this year or previous years, so you can determine what classes you may want to enter your birds.
Usually the show will be broken down into a commercial and an exhibition division, for chickens anyway. Generally the exhibition division is divided simply into 4 classes: hens, roosters, cockerels,and pullets...The judging is based on each bird being compared as to how close it conforms to the breed standard. There will be 4 class winners that will compete for grandchampion and reserve Grand champion. So the class champion in each class should be the best hen, the best rooster, the best cockerel, and the best pullet,determned by each being as close to the standard as possible. You generally don't have indivdual breed classes, just age/sex classes...Again each state is different...Again ask questions of your leaders. The best bet would be attend the show to learn first hand before you actually show a bird yourself, but learning on the fly is OK too.
Finally there will probably be a showmanship class. It may be broken down by experience levels. In it you will be judged by your general knowledge of your bird and how well you handle it around the judge. 4H shows are supposed to be fun....You will do fine, I'm sure.
First of all, your birds need to be hatched after January 1st of this year. You need Pullorum papers in order to even get them in a cage at the show. They need to be mite/flea free. The birds need to be purebred, not any kind of cross-strain.
In my county, we have laying pens (two females), meat pens (not sure the requirements, I don't do this category), bantam breeding pens (one male and two females), and standard breeding pens (one male and two females). For the breeding pens, they judge the rooster as half the points and the two hens as the other half. The two hens need to look almost exactly the same (in size, weight, comb, coloring, etc). Then they judge on quality.
Make sure you get your papers in on time.
For showmanship its generally about how you handle your birds and your knowledge about your birds. Make sure you know anatomy and feed requirements and your birds hatch date because those questions come up a lot. Oh and know your diseases.
I LOVE showing chickens
I just went out to look at some Leghorns for my first laying pen. Yay