The only dumb questions are the ones you don't ask.
Yes, I do post riding western. I have never had a riding lesson in my life, so I'm not sure how correct it is, but you do need to move with the horse when riding, or you will get very tired and sore. At a walk and a lope you should sit, but at the trot you will find it easier if you do a slight "sit, stand, sit, stand" with the motion of the horse.
I have taken groups of people who have never ridden before on trail rides, and they often ask what is the best way to ride. I have found that posting is very hard to describe to beginners, but after a rider has spent hours in the saddle it just comes naturally. I think if you ride at a trot for very long at all, you'll figure it out.
The trot is usually very uncomfortable if you try to sit, unless of course you're riding one of those Western Pleasure "shufflers", lol!
Anyway, it will come to you as you ride, or perhaps you'll take some lessons and your trainer can help you.
Don't worry about asking questions! That's what this forum is for!