Your posture -
You should sit straight and tall in the saddle with your shoulders slightly back. There should be an imaginary lateral line that connects your ear, shoulder, hip, and heel. Your heels should be down and pointed forward and your toes or heels should not dig in the horse's side. Your knees should point toward the ground.
Posting Trot -
It's really easy once you get the hang of it! You ride with the rhythm of the horse's trot, slightly "standing up" (for lack of better words) in the stirrups. Remember not to sit back down hard or else you risk hurting your horse's back. Also, less is more, your behind barely has to come out of the saddle to post.
Stirrup Length -
A general rule of thumb to measure stirrups is to stand on the ground, ball up your fist and put it on the stirrup buckle when the stirrup is in the position you would ride in, then (with your ARM STRAIGHT), pull the stirrup iron to you and it should come to about the middle of your arm pit. If it doesn't, lengthen or shorten the stirrups until it does. Do this for both sides, as one leg is typically a tad longer than the other. Also, depending on your age, your body may not be proportioned exactly right, so further adjusting may be needed once mounted. Also keep in mind that you mount on the left side and the added pressure to the stirrup leather causes it to stretch, so it may need to be shorter than the right side. And switch your stirrup leathers periodically to keep them equal length and in good shape!
I don't show anymore, but when I did, you always have to keep in mind that all horse people are NOT the same. And sadly, there are some very cruel and rude people out there. In English riding, appearance is crucial, so make sure your tack, horse, and yourself is clean and shiny (you don't have to literally shine).
English saddles are much smaller than Western and your Western trained horse may not be balanced under an English saddle. Make sure to anticipate having to help him learn to balance with the English saddle.
Good luck making the switch! :)