First, remember that your eye is the rear sight. You have to place it in the same place with regard to the rest of the gun every time to avoid a parallax error when using the scope. So...what is parallax?
Parallax is an apparent displacement against a background, or a difference in orientation of an object, when the object is viewed along two different lines of sight. Parallax is measured by the angle or semi-angle of inclination between those two lines. In a riflescope, parallax is an optical illusion. Parallax occurs when the "primary image" of the object is formed either in front of, or behind the reticle (crosshair) of the scope. When you move your eye from its proper alignment with the scope, the resulting parallax moves the image in relation to the crosshair, causing your aim to be off.
Think of it this way. You're sitting in the passenger seat of the car and you look over at the speedometer. It will read differently to you than to the driver, because you're not lined up with the steering wheel and gauge in front of it, so you're not getting the true reading.
Every scope has a quality called eye relief. That's the distance behind the eyepiece lens that your eye should be placed to be able to see through the scope effectively. You have to place the cheek of your shooting eye against the stock; move your head forward and backward along the stock-always with your cheek against the stock-until you get the best view through the scope.
The best view is when sight picture in the eyepiece lens fills the entire lens. As you move your head forward from the best viewpoint, the picture collapses, and when you move your head back from the best viewpoint, the picture starts to get smaller and then goes black.
So, practice getting the same cheek weld every time you shoulder your rifle and you will be one step further in taking a good, clean shot.