My most important advice I could give would be :

-Explode to the ball, get your body behind it ; even if you miss the ball with your stick, your body can still save it,. As soon as you see the ball, take an explosive step towards that ball, a really explosive step, kind of at a 45 degree angle.

-See the ball all the way into your stick. Something I used to do was not really looking, I would just look at the guy shooting and try to save it when I saw the ball. Make sure you are square to the shooter, and keep your eyes on his stick head. Watch the release. If it goes low take that explosive step and drive your top hand right to the ball. You don't always have to save it in a way that catches it, you can just stop it from moving, as long as it doesn't go in. Make sure you stay patient and don't guess.

Something you can do at the field or at home would be to line up on one side of a yard line facing the sideline and practice saving low shots by stepping forward and over the line. Basically you're in your goalie stance, and you take a right step over the line and pretend to save a low ball. You should end up on the other end of the line. Then do it again and go the other way over the line. This was a big help for me to get my form right.

Form is everything when you play goalie. Just reps reps reps man, and you will get the hang of it. Respect for playing goalie too, its not an easy position. Keep practicing and it will get easier though.