This would have to be the best exercise for developing all three parts of your shoulders namely the anterior delts, the medial delts and the posterior delts.
Take note; A prerequisite for any lifting where your arms rise above the shoulders is rotator cuff exercises so unless you have already been working on these then put some effort in to your rotator cuff before getting too far with shoulder presses. And always remember to do a full set of basic stretches before any kind of lifting and that includes the legs.
Practice first with a 20kg Olympic bar, a lighter pre-loaded bar, or an aerobic bar. Extra plates can be added as you progress. Alternatively use a Smith Machine although, unfortunately, the Smith machine will not allow the bar to be directly over your head which is absolutely necessary to get the best result. If the bar does not travel up and over your head the rear delts will not contract as they should for this exercise. The medial delts will not get the full benefit either. Try using a rack to support the bar at chest level as this saves energy for the prime muscle group instead of using energy to get the bar up to your chest.
Beginning with a standing shoulder press pick up the bar standing with your feet shoulder-width apart. Suck in air, tighten your core, and using your core, forearms and biceps, reverse curl the bar up to your chest. Relax and check your posture to ensure your back is straight and your head is in line with your back.
Begin the lift by simultaneously sucking in air and tightening the core. Then, keeping the core tight, and with a smooth motion press the bar up blowing out air to help keep the core tight throughout the lift. If your lungs remain full of air you will not be able to keep the core fully contracted throughout the lift. Always suck in air and tighten the core just before the lift and blow it out on the concentric movement.
The main muscles used to lift the bar are the delts and triceps. The line the bar will take is from your chest in a line straight up, then tilt your head slightly back to allow the bar to continue straight up and back finishing in a position directly over the centre of your head with your head in line with your back. This will ensure the load will centre over the medial delts with some of the load on the posterior and anterior delts. Your focus throughout the lift will be on the posterior and medial delts. The anterior delts will take more load at the start of the lift but also some throughout the lift. Be sure to follow this line to give an even balance of load over the three delts. About half way up the triceps will take 50% of the load. Hold at the top for 2 seconds.
If the bar is too far forward the anterior delts will take too much of the load and the posterior delts taking little or nothing.
Do 3-4 sets of these and then do 3-4 sets with the bar behind your head following the same method. Begin this by lifting the bar over and behind your head. When the press begins and the bar goes up tilt your head slightly forward and then back in line with your back. Remember, as the lift is completed the bar will be centred over your head.
Performed correctly this routine will develop a very strong and even balance of powerful muscle over the shoulders.