know your parts and history of the breed. Know the Standard for your breed and color. Use a calm bird in good condition, but the bird needs to be active enough to pose a bit for you. Be confident, but not cocky. If you don't know the answer, tell the judge you don't know; generally they will ask you a very hard question to try to stump you. Be very specific and don't be afraid of talking too much. Also, Be sure the head goes in the pen first and comes out of the pen first!! This is probably the most commonly made mistake in showmanship.
In a nutshell, that's it. Atire is a white lab coat/white shirt and black pants, or generally nice clothing. I prefer a lab coat, although it can be difficult to get your hands on one. Also behavior is important. Smile! (I always get too nervous to smile). Maybe try come up with an intro, just in case your showmanship judge would like you to introduce yourself. I keep memoried: "My name is _______. I have been in 4H for ___ years and this is my _____ year exhibiting. This is (Name here, if the bird has one), my variety, then breed."
Know the bird's origins and what they were bred for/how they came to be. What was their purpose? Also know some general poultry knowledge. Paracites, flock management, how you would help other exhibitors, what you feed your birds, etc.
If the judge does not give you the opportunity to say something or add something, best not contribute. Some judges will ask if you have questions, others will not. You can ask questions at the end, or ask if the judge can meet with you after he/she is donw with the other exhibitors. Also, try to memorize all of the parts of the body that you can, but if you are stopped early or are told not to continue, don't insist on continuing. It always bothers me when I show and I only get done with the head before the judge tells me to stop. I've been tempted once or twice to ask if he/she was really SURE they didn't want me to keep going on, but you can't argue with a judge's judgement so that's that.
There's alot to know. But BYC is the best place to start. Don't worry about knowing everything. My first year was just to see how it worked, then I began to catch onto things and find out more later. I was honestly lost my first time.
The pubic bones are the two 'bumps' that you can feel under the vent. Feel with your fingers and you'll feel two sharp bumps, it'll feel like they're sticking out of the bird, but that's normal. How far they are from each other depends on the bird's size (bantam or large fowl), and if they had started laying.