I would also suggest getting a trainer to help you. The concept is pretty easy, but takes a bit of practice to get the rhythm down and keep the horse moving. I'm just starting out, but counting "one two" out loud on the up/down beats really helps. Keep your heels down, don't post too high, and squeeze at the top. When I started, I worked on posting holding the saddle, the horse on the lunge line. Worked to posting with my hands on my hips, on the lunge line. Now I'm posting off the lunge line holding the reins. The hardest part for me as a beginner is remembering to keep my hands low, I tend to draw them back and accidentally tell my horse to slow or stop.
For me, when posting to the trot, I had to learn to feel it. People have told me to look down at the outside shoulder, and as that shoulder/leg pushes into the ground, I should rise up. I get all sorts of screwed up following this.
I know when I'm posting to the wrong leg, because I get a twisting motion in my post (as I ride, my body twists to the left by nearly 40 degrees and my legs start swinging). I know when I am posting to the correct leg, I get a double bump when my butt hits the saddle, and if I am riding strong, my body does not fight the straight, forward motion of the rise.
So you have to learn to feel it out. I started taking lunge lessons with my eyes closed so I could ignore everything but the feel of the horse under me to understand how the horse moves under me. But I never actually got a great post down, until I started cantering. I never got a great canter down until I started jumping. I'm very backwards, I know.