Lets start with a four-seam fastball. When you hold the ball, you should hold it so the lines (seams) make a letter "C". You take your index and middle fingers and put them on top of the seams with your thumb on the bottom. As a beginner, you typically use your index, middle, and your ring finger to give you more control. Next, when you release the ball, you want to come over top and flick your wrist down, letting the ball roll off of your fingers. This will create backspin on the ball. The pitch is meant for a high velocity pitch to set up your offspeed "junk" for later in the at-bat.
The two-seam fastball. If you are looking down at the ball, you should have two seams running from bottom to top. Then put your index finger along the seam on the left, and your middle finger on the right. Throw the ball just like you would for a four-seam and the ball should slice a little bit left, right, or down.
Next, there is a cut fastball. Take the ball like a four seamer. Turn it untill the ball makes a bacwards letter "c". Then slant the left side up at a 45 degree angle. Snap your wrist down while applying pressure to your middle finger. This should make the ball cut into or away from the batter while maintaining a high velocity.
Finally, the split-finger fastball. This is like a two seam. You hold the ball like a two seam, then turn it down a little so the seams make sort of an "n" shape. Then put your index on one side and your middle on the other side at the widest points. Usually younger kids cant throw this pitch because their hands are to small. When you release, flick your wrist down. When the ball arrives to the plate, it will dive or drop to the ground.