First, did you have any expenses that you incurred as a volunteer at the church? Any equipment that you had to buy, books that you purchased, travel that you had to pay for? If so you should be deducting any of those expenses from the $18,000 in order to reduce your tax bill.
Second, self employment tax on the money that you have earned is 15%. You need to be sending in at least 15% of what you're being paid each month (minus any expenses) to cover that. If that is all the income that you make for the year then 15% comes pretty close to taking care of the tax bill.
@Kmruda
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