How to you shoot while driving on grand theft auto?

On grand theft auto 4 for 360. It says press LB but it doesn't focus on the enemy and I end up wasting all my ammo trying to hit him. I can't press LT because that's the brake? How do I aim and shoot at an enemy while driving?How to you shoot while driving on grand theft auto?
The reticle doesn't lock on anything like it does when you're on foot.



To make it more difficult, you have to move the reticle with the analog stick, then hit LB to shoot.How to you shoot while driving on grand theft auto?
When you press LB in a vehicle it doesn't lock on to an enemy to make it more difficult, you have to use the reticule on your own to aim.How to you shoot while driving on grand theft auto?
LB the only way to aim while your driving, it doesn't lock on to any one