You may not even realize it, but you may be gripping the pistol tighter before each shot in anticipation of the recoil. Even a slight increase in grip strength applied right before your shot will equal shooting low on a target. Try to keep you grip steady and constant.
Sounds like you've got the right sight alignment now you just need to improve that grip. However, remember since your new it doesn't hurt to be overly percise in your shooting stance, grip, breath control and trigger squeeze.
CHECK OUT THIS WEBLINK FOR SOME GREAT INFORMATION ON PISTOL MARKSMANSHIP
http://www.globalsecurity.org/military/ ... /chap2.htm
If you still have trouble on your own after three or four shooting sessions than take a look into taking a shooting instructions course from an NRA certified instructor. Most gun shops have flyers for local shooting courses or know of them if you ask. I always find its best to learn the right way first than have to try and correct bad habits later.
NO ONE gets his/her first pistol, walk on to an IDPA course and shoots like a pro without having gone through the basics. Even if you've had experience in the military or long ago with pistols it doesn't hurt to take a refresher.
Take one thing at a time and take your time! Best advice I can give.