More than anything, what distinguishes a good infielder from a mediocre one is the ability to concentrate. In baseball, there is a lot of time for the mind to wander, but a good fielder won't let himself get distracted and thus miss a play.
You should be concentrating fully by the time the pitcher starts his windup. It's not important to follow the pitch, but it helps to know what pitch is being thrown so you can anticipate whether the ball will be pulled or hit to the opposite field. I used to lean slightly in one direction, not really committing myself, but doing a little walk sequence to give myself momentum, much like a tennis player returning serve.
In catching the ball, have your hands extended comfortably while you "look" the ball into your glove. Many young players pull their heads away when fielding a hard grounder, but you'll seldom get hit if the ball takes a bad bounce: it usually goes over your head.
Too many infielders forget to keep their glove low, and the ball often goes under it. Don't stab at the ball unless you can't get to it any other way. Have your glove down and open well before the ball comes to you (see photo). The fewer movements made in fielding, the better.
You will have to learn to throw from all positions. If possible, throw overhand or in a three-quarters motion. There will be times when you'll have to throw underhand or sidearm, but the ball has a tendency to sail then.
Remember, too, that sometimes the best play is no play. When I first started at Baltimore, most of my errors came on careless mistakes, such as throwing when I didn't have to. Be alert and you will reduce errors significantly.