the video that tihannah posted is good! it kinda just goes through all the basics. if your first learning though, do not pay attention to diagnals. you should just get the basic movement down first. counting is also very helpful, "one two one two.." with the horses motion. i also found it very helpful when my trainer counted to me, instead of me counting myself - it was just one less thing for me to really think about. you kind of just really have to feel it and go with the motion of the horse. i assume you have a trainer or someone more experienced helping you? could they put you on a lunge line so that way then you only have to concentrate on posting, not where the horse is going and everything else? was also very helpful for me to watch other riders. just watch them while they ride and really pay attention to what there doing and how there riding.
it will feel weird and be hard at first. just dont try to over do it or over exaggerate it. the post is really just a small motion. you should stay close to the saddle, have the same height of post, and hit the same spot in the saddle each time. while your first learning though you are expected to be bouncy, rise really high one time and then barely at all the next. also, every rider has there "posting epiphany" as my trainer calls it, one day it will just click and they will get it and be good forever. it just takes a lot of time and practice. my friends little sister finally just got it, it took her over at least six months of weekly lessons (shes only 6 though). it does take some people longer then others. just take your time and think about it, but make sure your not over thinking it either and getting yourself frustrated.