Targeting
Using the Torso
The first thing you have to learn is how to twist the torso so you can track targets. As I said in the piloting section, I like the mouse for this. But use whatever feels right to you. In all mechs you can swing the torso freely and track whatever you want "Torso Twisting". The larger mechs have a limitation as to how far their torso may twist. Usually 180 degrees is the maximum that they may twist. However some smaller mechs have a free torso, i.e.: they can twist a full 360 degrees. This will allow you to actually run away from the enemy's mech while still targeting it! Twist shooting is ABSOLUTELY VITAL, you cannot get along without it.
Leading the Target
Like I said in the lag section. Occasionally you must lead a target because of internet lag. The instances where you never need to consider lag would be.... When a target is stopped.... Or if it is running straight at you, or straight away from you.....Or if you are shooting a locking missile.
Hitting with missiles
Hitting big slow mechs with streak or LRM missiles is pretty easy. Wait for a tone and cut loose. Enough said. But fast mechs require a more complex approach. LRM's are not good for close distances. They will nearly always miss. Use Streaks or SRM's up close. When using streaks, when you get a lock, just fire. With SRMs you must lead the target - similar to lag shooting. Try to anticipate which direction the mech is heading and aim your missiles at that point. Usually it will be around one mech's length in front of a fast mech. Again, practice will help you to learn where to shoot.
The Guns OF Choice
Lasers and locking Missiles are by far the easiest to hit with. The rest of the weapons may require some lead. The only other weapons I consider are Autocannons and LBX's because of the massive damage they do. But they are close range only guns.