When trying to learn how to jump higher and dunk the basketball, there are many factors to consider. If you do not have the innate ability to be able to jump three feet in the air, you will need to work out. The most overlooked thing athletes do when training how to jump higher is working out the total body. Just doing squats all day will not improve your vertical leap to the level you are looking for.
Although having strong legs is an advantage when training to dunk the ball, you need to condition the rest of your body also. Look at Olympic power lifters; you think they can get that enormous torso off the ground to dunk a basketball? They undoubtedly have some of the strongest legs in the world but their upper body are not for jumping.