Find a strong plastic bag that is large enough to fit around your showerhead. A garbage bag works well!

Angie's How To Unclog Your Showerhead
Limescale and mineral buildup can clog water passages over time and decrease water pressure. This easy, nontoxic approach allows you to unclog your showerhead without removing it.
3 pros!
Mix 1/3-cup baking soda with one-cup white vinegar in the bag. The baking soda will react with the vinegar causing it to bubble, so we recommend doing this in a sink.
Secure the bag over your dirty showerhead with a rubber band. Make sure the head is completely covered in the solution. The acid in the vinegar reacts with sodium bicarbonate (in the baking soda) to form carbonic acid, a strong cleaning agent.
Leave the bag with mixture on the showerhead overnight.
Wipe down the showerhead with water until the solution is gone and run the water for a minute before you step in.
BONUS: You can also grab a pin or needle and gently poke it through the showerhead holes to clear the passageways.