In general, you can deduct money donated to the church. If you donate more than $75, the church is required to give you a receipt. You can deduct money actually donated when it is donated, a promise to donate later is not deductible until you actually donate the money.
Directed donations are slightly tricky. You can't control the use of the money. For example, if you donate to repair the church roof on condition that the church hire your brother's roofing company, that would not be allowed as a tax deduction. But if the church decides on their own to repair the roof and sets up a special or separate fund, your donation is deductible whether you donate to the general fund or the special fund.