Remove the Showerhead
Using adjustable pliers wrapped in a heavy tape, unscrew the nut on the showerhead and remove it.
If possible, disassemble the showerhead and use a wire brush or a paper clip to clean mineral deposits from the various parts.
Soak in White Vinegar
Soaking the individual parts or the fully assembled showerhead in white vinegar overnight will dissolve mineral deposits and help get the head flowing properly again.
Reassemble
Apply plumbers tape or silicon grease to the threads on the shower arm before reinstalling the showerhead.