The basic baseball grip is the four-seam or across seam grip. The four-seam grip is used for throwing fastballs that will stay up longer and move in a straight path to the target. The more spin, the faster the ball cuts through the air.
Use the four-seam grip for infield throws to bases and cut-off throws from the outfield.
To get movement on your fastball, try a two-seam grip held with the fingers along the seams. The ball will drop and move when thrown well.
Young players often use all of their fingers to hold and throw a baseball. Some players call this the three finger grip or box grip - used for the change-up by older players. These throws are slow, but more accurate for young players with small hands.