The fastball is an effective pitch, even if it's not overpowering. More important is control and good location.
Know your hitters.
Know their weak spots.
Know where to pitch them and when.
(A good catcher helps, as does good information about the hitter, hopefully in the form of previous exposure.)
As far as how to throw it, there are basically two ways:
The four seam fastball is called such because the pitch uses all four seams to cut through the air. Thus, you grip the ball across four seams, as shown below:
When you throw a four seam fastball, you don't want to throw it completely overhand, but more at "three-quarter," which is slightly more sidearm than straight overhand. When releasing the ball, you want to snap your wrist to produce as much backspin as possible. This will cause your pitch to seem like it is rising. It does not actually rise, but seems to because it is not falling as much as the batter expects.
Each pitcher's windup is different, but one possible arm motion is displayed below:
One of the keys to throwing a fastball properly is to keep your palm facing away from the plate during your arm motion, as shown in the first picture of the sequence. You don't want to throw it like a grenade. Also, be sure that your elbow stays above your shoulder during your motion, as shown in the second picture. When your elbow drops below your shoulder, you lose a lot of power. A good follow through is also important, though not pictured. You don't want your arm to slow down before you finish throwing. A good follow through, where you throw "through the pitch," prevents that from happening.
The main difference between a four seam fastball and a two seam fastball is the grip. Instead of holding the ball across four seams, you hold it accross two, as shown below:
Since a two seam fastball has only half the seams cutting through the air, it doesn't "rise" as much. Many coaches have told me that a two seam fastball has more speed than a four seam fastball, but the trade-off is that most pitchers suffer worse control when throwing a two seam fastball.
One benefit of throwing a two seam fastball is that by putting pressure on the ball with either your index finger or middle finger, you can cause the ball to "tail" (curve slightly) in one direction or the other. If you put more pressure on the ball with the finger on the right (for righties, your middle finger), the ball tends to tail right, and by putting more pressure with the finger on the left, the opposite occurs.
The windup for a two seam fastball is the same as with a four seam fastball.