One of the common things for all players to do much of the game is to try to take away the ball from an opponent. Most youth players will attempt to stab at the ball hoping to nudge the ball and steal it that way. The problem with this method is that it is not a good way to approach a steal, especially if the opponent has good ball handling skills.
What a defending player needs to do is pay particular attention to two things: the ball and the opponent's feet. Obviously it is the ball that is the object of the steal. Timing of the steal is what is difficult. The opponent's most vulnerable time is when he/she is off balance. That time occurs when one foot moves the ball and the other foot is off the ground on the move. It is at that precise moment the ball can be taken.
I teach my defenders the "two yard set," a method that if followed well, increases the chance of being able to first slow down an offensive player with the ball, and second steal the ball away as the opponent is off balance. I explain it as the "Two Yard Set," to emphasize that there is some distance between the opponent and the defensive player in this case. Somewhere between one and two yards, and preferable on the imaginary line between the opponent and the goal, the defender "sets." To set, a defender is some what crouched, knees bent comfortably and balanced, heels slightly off the ground. The eyes are on the ball with recognition of the opponent's feet movement.
The set is then held till the opponent starts a dribble. As soon as the opponent gets off balance (when he/she is on one foot and moving the ball, the defender moves in for the steal. If timed properly, the distance between the players gives time to react without the opponent racing around the defender, and because the opponent is off balance, allows for an easy steal.
Defenders must practice this as much as possible. It is a critical part of the game.