Most people use a high-g accelerometer to detect and measure forces. I doubt very much that a force-sensitive resistor would be useful as they are not quantitative.
In either case you also need a microprocessor, a battery and some way of storing and communicating the data, so this is an advanced project.
Do the research and groundwork before worrying about how to get stuff into a baseball.
You also need to be clear on some basic concepts concepts in physics and measurement: the force accelerating a baseball while being thrown has little to do with "how hard the ball is being gripped".