If your showerhead sprays unevenly, take it apart and clean it, or replace it. If it leaks at the arm, or if it doesn't stay in position, tighten the retainer or collar nut. If that doesn't work, replace the O-ring--or replace the showerhead.
If you want to replace your showerhead, take the old one with you to your supplier to make sure you get one that will fit. You'll find a wide range of styles and features.