If your shower head leaks where it meets the arm, you probably need to replace the washer.
To reach it, loosen the collar, using tape-wrapped rib-joint pliers. Unscrew the head from the adjusting ring.

Erratic or weak pressure usually indicates mineral buildup.
To restore proper flow, clean outlet holes with a pin or unscrew a perforated face plate and soak it overnight in vinegar, then scrub it clean.

If the shower head pivots stiffly, check the washer for wear and coat the swivel ball with petroleum jelly before reassembling.