Dear lets talk pitching,
First, let me put out the manmdatory cautions: youngsters should not throw breaking pitches, their arms are not ready for them.
You throw the sinker pretty much like a fast ball. It is mostly a modified two seem fastball. Take your two seemer grip, then move your pointer and middle fingers closer together. They should now be between the seems (fingers on seems takes away the sinking action). Offset your thumb from the 6 o;clock position to about 7 or 8 o’clock.
When you throw, this grip will make you, without trying, “turn the ball over”. From a righthander, the ball will have a little clockwise and “top” spin to it. The ball will TEND to move in and down to a righthanded batter.
Move your thumb the other way and you will be throwing a “cutter”. A cutter is a fastball that acts like a slider (without as much movement and sinking action) but has almost the velocity of the two seem fastball.
The regular two seem fastball will tend to tail into a right handed batter from a righthanded pitcher.
Another way to throw the sinker is to widen your pointer a middle finger a bit from the two seem fastball. Now they will be outside the laces of the “sweet” spot on the baseball. Thumb stays in the same position as a regular two seem fastball. This pitch will tend to break straight down late. You will loose velocity (compared to the two seem fastball) but pick up movement down. With a little pressure from either finger, you will find the ball tailing one way or another. Some call this a sinker, others call it a split fingered fastball. Neither term is completely accurate but both suffice.
If you have large hands and widen the split in your fingers so that the ball rests on the web between those fingers, you will be throwing a fork ball. That pitch breaks down hard and is fairly slow. It actually works like a poor knuckle ball but at a higher velocity.
Again, you need a coach right there with you that knows what he/she is doing. You should also not be trying to throw these pitches until you are at least 13 and more like 16. Some people say that widening your fingers on a ball can hurt your arm. I say it depends on the size of your hands and length of your fingers. Be safe with it.