First of all, as always, SAFETY is paramount. As has been already stated, you can keep a shell in place while waiting to shoot, but don't close the action until it's your turn. I typically close the action on my gun as soon as the person preceeding me pulls their trigger.
The game is played on a field with a single thrower and 5 stations, numbered from left to right. For beginners, you should be shooting "16 yard trap", for which you use the line closest to the trap house. Inside the trap house is an oscillating thrower (which you cannot see). When you call "Pull!", the thrower will release a clay target on one of 5 angles, but with a consistent trajectory (unless we're talking about "wobble trap" -- unlikely).
All of the shots will be taken at rising targets which are moving away from you... kind of like simulating hunting pheasants over a dog. Since there are 5 stations and 5 angles at which the targets can be thrown, there are theoretically 25 different presentations you can see. In reality, they boil down (roughly) to the following:
* straight-away
* slightly left
* slightly right
* moderate left
* moderate right
* hard left
* hard right
Straight aways and slightly left or right really don't require any lead. The moderate presentations will require that you at least get past the leading edge of the bird. The hard lefts and rights will require some actual lead. All of this is variable based on how fast you move your gun, what your follow-through is like and such. I can't tell you how much lead to use... but as a rule of thumb, beginners most often will shoot behind the bird. If you are missing on the "crossers", try increasing the amount of lead you have and see if that helps.
Try to focus on the target. While others are shooting, try to visualize the absolute front edge of each target. Don't look at your bead. Look at the target. Shoot early, while the bird is still rising. If you wait, then it becomes a longer shot and the trajectory will start to change. Trap targets which are easy at 40 yards become very dificult when dropping at 60 yards.
A single round of trap consists of 25 targets for each member of a squad. At a minimum, I'd recommend that you take 27 shells to the line. You never know when you might have a bad primer, and it's in good form to be prepared. After your 5th shot at each station, wait until the trapper says "Move" and then proceed to the station to your right. If you are on station #5 (the one on the far right), proceed back to station #1. When doing so, make sure that you keep the barrel(s) pointed in a safe direction and the action open.
As far as where to point the gun when you call for the bird... BozemanMT described one method. I use a little different approach. I aim at the roof of the trap house, about 1 foot down from the front edge (I'm calling the front edge the one where the clays come out of). Essentially, I'm aiming at the roof. From stations #1 and #5 (the end stations) I aim at the nearest corner of the house. At station #3, I aim at the middle, and from stations #2 and #4, I aim about halfway between the middle and the corners. Try it each way if you want and see what works best.
Be safe. Don't worry too much about it. Once you see a squad shooting, all of this will start to make sense. Enjoy yourself! It's an addicting little game.