If your shoulders hurt, your working surface is probably too high. It's the most common form of ergonomic stress.

Sit in a comfortable armless chair with the height adjusted so your feet are flat on the floor and hips bent not quite to 90 degrees.

Relax your shoulders, put your elbows at your waist and hold your hands palm down over your thigh, with your wrists straight.

Bend your elbows to 90 degrees.

Right UNDER your hands is where your keyboard should be.

Do whatever you have to to make it happen. Cutting the legs off your desk? Adjusting the chair higher and having a foot rest? Adding a keyboard tray?