Comprise means "is made up of" or "consists of." The whole comprises the parts.
Compose means "make up" or "make." The parts compose the whole.
Incorrect: The rock is comprised of three minerals.
Correct: The rock is composed of three minerals.
Correct: The rock comprises three minerals.
Correct: Three minerals compose the rock.
If you are confused, just say, "The rock is made up of three minerals," or "Three minerals make up the rock."