How to get TIFF file with 8 bit color depth

[CDB1983]

Hello. My goal is to submit a TIFF file with the highest quality possible to Nations Photo Lab for printing. The website for Nations Photo Lab says that the file submitted must have 8 bit color depth. I have read conflicting reports about the color depth of files exported from GIMP. My question is: as a GIMP user, what do I need to do in order to get a TIFF file of a picture with 8 bit color depth? Thank you.