Both jetiki and JMS have given you good advice. To answer your question about setting you horse up-
I've always been taught to hand-set a horse in a halter class. To do that, get your mare to stop and stand. Tell her whoa and hold on to her halter with your left hand as you kneel down on her left side. Using your right hand, pick up her feet ( you might need to work on this first if she's not used to it) and set them so they are even with each other and "square". I've also been taught that you should never move the right hind foot. Always use that foot as a guide to set the other hooves. If this is the mare I said was a little cowhocked, you'll want to gently pull her hocks to the outside to straighten her up. As you stand up, I suggest running your hand (with fingers on each side) down her back bone to her hips, pressing firmly. This will make her lift her but and make her tailset look better (if you have a horse that is already butt high you don't want to do this). Some people do use lead pressure to set their horses, wich works, but I've been told to always hand-set in halter. Our horses are trained to set off of lead pressure for showmanship, though.
To stretch her neck- once you are back standing up and in front of her ( for halter you want to make sure you have a step or 2 distance between yourself and her nose so the judges can see her head) you'll want to take your treat or feed or whatever out of your pocket and show it to her, but hold it out infront of her. You want her neck to go "up" and then "out". You can talk to her and make noises and stuff to get her ears up, but be pretty quiet about it in the ring.
Sorry if I overwhelmed you
I hope all that made sense. Are there any trainers around you or people that already show that could help you out? Once you see the whole process done, it's a whole lot easier than it sounds. Also, try going to a rated show ( if thats what you'll be showing) and watching some of the halter classes. That will really help.
This a picture of my halter gelding. This is an AMHR show, so they can be parked out a little more than in AMHA, but he is stretched out too much. In other words, his back feet need to be more under him.
Can you see how his neck goes up and then out?