Edit: beaten to it. I type too slow...
There are a couple of basic tests of fit before your start. If these aren't right you'll struggle to develop a technique. Get these basics right and then you have the foundations for a technique you can polish
Firstly, the pull length needs to be right. Hold you trigger arm out in front of you, slightly bent at the elbow with your fingers together and your hand flat and straight. Lay the butt of the shotgun in the crook of you elbow and let lie along your straitened forearm and rest on your hand. The trigger should naturally fall between the first and second joint of you index finger (front trigger if it is double trigger). If it falls at your finger tip or beyond, the pull length is too long and you will be straining to reach the trigger. If it falls closer than your second joint the pull length is short and you will feel cramped.
The second reference point is the drop at heel. That's essentially the angle of the stock relative to the barrel. This needs to suit your physique or the gun will never point where you are looking.
Practice mounting the shotgun. Focus your gaze on some object in front of you and above your head as though it were an incoming bird. Distance doesn't matter, you can do it inside the gunshop. The corner of the wall and ceiling will do.
Keep both eyes open, maintain your gaze on this spot and bring the shotgun to your shoulder. This is where it helps to have someone with expertise watching you to make sure your mounting the gun correctly and your feet and body position are correct. Keep doing this til it starts to feel natural. Basically, when shooting you track your target without ever taking your eye off it, and when you mount the gun the trajectory of the shot needs to fall naturally into this plane without you having to adjust your body to accommodate it, by which time the bird has gone, or you've taken a hit-and-hope bang and missed it. You acquire the target with your eye and you don't want to have to take your eye off it to find it again with a gun barrel that is pointing somewhere else.
When you're comfortable with mounting the gun to the shoulder, do it again and hold, and without altering your posture shift your focus away from your imaginary target and look at the view the gun is presenting down the barrel. All you should be able to see is the bead sitting neatly above the top lever, so that you are looking perfectly along the rib of the barrel. If you can't see the bead at all the gun has too much drop at heel for your and the barrel is diverging downwards from your line of sight and you will miss low. If you can see the entire rib as though you are looking down on it from above, the gun doesn't have enough drop at heel and the barrel are tilting upwards away from your line of sight and you will miss high.
Hope that makes sense and isn't teaching you to suck eggs.
There's a whole load of other factors involved in shooting a shotgun proficiently, but if the above are right you can learn the rest with practice and tuition. If they are not you will always struggle and become frustrated.
