It is a percentage of your income tax and depends on the federal state. Membership is recorded when you register your address, so check what you answered on the Anmeldung form.
Leaving the church is done at the local registry office or court, not at the tax office. Church tax paid is deductible as a special expense.
Full guideChurch tax in Germany — and how to stop paying itRelated terms
General information on German tax law, not advice on your individual case.