How can I stop taking zoloft (sertraline)? I'm on 50mg have been using it for 9 months?

Under supervision. Discuss it with your physician to see what is safest for you. However 50mg is low enough to stop it without having to decrease the dose.
Decrease dose. Under the supervision of your physician, the best way is to cut the dose in half and do this gradually over a couple of weeks. Ask your doctor or pharmacist if it ok to cut the type of pill you are taking in half to get to 25mg for a couple of weeks. After that, you can stop it.