Turret View is an extremely useful control mode for vehicles, where your camera no longer automatically faces forward, but is instead moveable around. What's more is that your turret will move with the camera, giving you a 360 degree field of vision and firing. It is strongly advised you use this over the default mode, as it will make many tasks much easier.
 How to lock the vehicle camera to the turret?
Step One: First, hit ESC, and then click the "Options" button at the bottom of the screen, located between "Help" and "Suicide".
Step Two: Click "Controls", located at the top of the list.
Step Three: Click the "Look/Mouse" tab, located on the left side of the screen.
Step Four: Check the "Lock Vehicle Camera to Turret" checkbox, the lowest of the three checkboxes.
If you need further guidance, please refer to the images uploaded courtesy of Crimson. You can also revert back to the original camera mode by holding down the "V" key.