If your shower head leaks where it meets the arm, you probably need to replace the washer. To replace the washer, loosen the collar using tape-wrapper rib joint pliers to avoid damaging the finish; then unscrew the head from the ring and remove the old washer with the new.
Weak water pressure from the shower head usually means there is a mineral buildup in the shower head. To remove buildup and restore water flow, clean outlet holes with a pin or unscrew a perforated face plate of the shower head and soak it overnight in vinegar. Once soaked over night scrub the face plate clean and rinse off.
If the shower head will not pivot with ease, check the washer for wear and tear. If the washer is fine than coat the swivel ball with petroleum jelly before reassembling the shower head.
Tip:
If the nuts and couplings are frozen in place, instead of using a wrench to force them loss, place penetrating oil on the connections. Wait half an hour, then loosen the nuts and couplings with a wrench.
Make sure you always have handy the tools to help with plumbing repairs. Most are need for the repairs that are mentioned above for fixing a faulty shower head.