There are generally two primary styles of rolling the ball down the lane. Most newer players play by rolling the ball straight. More experienced bowlers usually roll a hook, which means that they make the ball start out straight and then curve towards the pocket. To produce a hook the player needs to let go of the ball with his thumb first, then the middle and ring finger release almost simultaneously.
Of course there are innumerable variations in style and technique and the position of the thumb can vary from person to person. The conventional bowling styles use either a four or five step approach beginning 8 to 16 feet behind the foul line. Some extremely young or physically challenged players may use both hands to swing the ball forward from in between their legs. This technique is easily picked up by weaker players but is seldom used because it is frowned upon by the bowling community due to the potential to damage the lanes and/or ball. Of course in time, with practice, you will achive better technique and results.