The first thing you need to concentrate on is proper sight alignment. With the open iron sights supplied on some rifles, you have three separate sighting elements to contend with: the rear sight, the front sight, and the target. To aim with open sights, place your cheek snugly against the upper ridge of your rifle's buttstock and look through the V or U shaped notch of the rear sight. Elevate or lower the barrel slightly until the top portion of the front sight blade can be seen. Then further adjust the sight picture until the front sight blade is centered in the rear-sighting notch, and the very top part of the front sight appears flush with the top surfaces of the rear sight on either side of the notch. While maintaining this front sight-rear sight alignment, move the rifle until the front sight covers the target or sits just under it. Most target shooters like to see the bullseye, just above the front sight post. When the bullseye appears to sit atop the front sight in this manner, you're employing what's known as a "6-o'clock hold."
Once you're familiar with your sights and the proper sight picture, it's time to refine your breathing and trigger control. This should be practiced while you're in the prone position with the rifle steadied by a rest (you can use a rolled up sleeping bag in a pinch)
From here on out I am going to assume you will learn on a rifle.
Set up your target and your shooing position for shooting from the prone (I like to lay on a shooting mat) set up you rifle rest (sleeping bag if you need to). Assume the prone position by lying on your stomach at a slight angle to the target. Right handed shooters should lie on a line with their head pointing to 2 o'clock on an imaginary clock face where the target is at the 12 o'clock position. Once you're down, spread your legs a comfortable distance apart and turn the inside of each foot toward the ground. Grab the rifle's forend with the left hand, then allow the forend to rest on the rest (rolled-up sleeping bag). Grab the pistol grip of the buttstock with your other hand, and allow that elbow to rest on the ground to one side. Pull the buttstock firmly into your shoulder and snug your cheek against the stock. The butt should be resting in the area formed by the juncture of your shooting arm and your upper chest-not on the muscle of your upper arm.
Adjust your position so the rifle's sights come into proper alignment with the target downrange, and stay there.. Adjust the rest (sleeping bag) until you can keep the sights on target with little or no effort on your part. The rifle should still be unloaded at this point. While maintaining the sights on target, place the index finger of the shooting hand on the trigger. Only the pad forward of the first joint should contact the trigger. A common mistake is to allow the trigger to rest against the first joint. This area of the finger lacks the sensitivity needed for proper trigger control.
Ensure the chamber is empty and the rifle is cocked. Disengage the safety. Now keep the sights in careful, continuous alignment with the target, and take a moderately deep breath. Apply a very slight amount of pressure to the trigger, and then exhale approximately half the air you've taken in. Hold the remainder of your breath while you slowly squeeze the trigger with the front pad of your index finger. When the trigger releases it should come as a surprise. If you find yourself anticipating the trigger break, you are likely flinching slightly or doing something else to move the sights momentarily off target. If you have someone with you, have him or her watch the muzzle of your rifle and tell you if it moves or jerks when the trigger breaks. There should be no movement when the trigger releases.
Don't underestimate the importance of this simple exercise. Without proper breathing and trigger control, there's no way you'll ever be an expert marksman. To many people overlook these simple basics and then wonder why they shoot bad groups (all over the place)
Now it is time to load your rifle and repeat the above with a loaded firing firearm. When you are done if you are following the fundamentals you should be able to shoot three shots and the three should form a tight triangle. After you have mastered this exercise with the rest it is time to remove the rest and perform the same until you master the off hand. It may take some time and quite a few rounds to accomplish this but ingraining good shooting habits in the beginning can lead to nothing but better things.