Generally, you would file as clergy. Are you an employee of a specific organization, either US or foreign, or are you considered a self-employed or independent contractor? Do you receive any US tax documents (like a 1099 or W-2) or not? Does "missionaries" mean you are married filing jointly, or are you single (a missionary)?
In general, any US citizen or green card holder must file a US tax return to report all their worldwide income, following US laws for deductions, expenses, and so on. You may also need to file a local tax return following local laws. If you do file a foreign tax return, you can claim the foreign tax you paid as a deduction or credit to reduce your US taxes. You report your income and expenses using the US$ conversion rate as of the day they occurred. If you don't have daily detailed records, the IRS provides a yearly average value you can use.
I'm not sure you can use Turbotax Online if you are overseas, it used to be you could not. It also used to be that you can't buy Turbotax to install on your computer (from a CD or download) if you are overseas. If these restrictions are still in place, you can get around them by buying a download of Turbotax to install on your own computer if you have a credit card with a US address, you may be able to get a discounted price from Amazon, Best Buy, or other authorized resellers. You will also need a US bank account for direct deposit of any refund or to make electronic payment of taxes due. If you don't have a US bank account, you will have to pay by check or get your refund by check mailed to you.
If you are self-employed, you need Turbotax Self-Employed online, or any version Deluxe or higher in the desktop product. If you are W-2 employees but you have expenses to deduct on schedule SE, I think you need to use the desktop program to deduct those expenses manually, it's not part of the standard interview.
You can post followup questions here on how to report income and expenses for religious employment. I sort of specialize in this area. Some (but not all, unfortunately) of the other "champs" and Turbotax employees can also answer questions on religious employment, it's a specialized area.
