Get Ahead in the Count
There are a lot of reasons for pitchers to get ahead in the count (that is, have more strikes than balls at any given point in the at-bat), but here's one of the best: statistically, batting averages are lowest when the pitcher is ahead in the count, and they increase when the pitcher is behind [source: McFarland]. Put another way, if there are two strikes and no balls, the batter has a far lower chance of getting a hit than if there are three balls and no strikes.
What does this mean for your pitching strategy? Your first pitch of an at-bat really needs to be a strike. For younger pitchers, most of time this means a fastball. You need to get it over the plate, get the batter to swing; if your fastball is moving, hopefully he'll miss. You can also start an at-bat with a breaking ball (this is somewhat rare), but you have to make sure it breaks into the strike zone. Throwing a ball on the first pitch gets you behind, and it's hard to catch up.
Other than the statistical advantage, why is getting ahead on the count so important? If there are three balls and no strikes, the batter knows the pitcher wants to avoid a walk. That means he'll be pretty confident the ball is coming over the plate. It removes a lot of the batter's uncertainty. It also gives him a cushion – he can take a monster swing at the next pitch because even if he whiffs, he'll get another shot at it.
At the opposite end of the spectrum, if you have two strikes on a batter and fewer balls, you can throw anything – a curveball, slider, a two-seam fastball, a changeup – and he'll have no idea where it's headed. You can try and pick the corner of the strike zone, because if you miss and throw a ball, you have that cushion to work with.