Turn off the main water at the bottom of the toilet and then flush it so it drains. Then spray a cleaner onto any stains or mold and scrub it off. Turn the water back on.
I've always poured white vinegar in let sit then just wipe it clean. flush then pour more. Let it sit until someone needs to use it. then use pour about 1/2 cup in about once a week before you go to bed. Let it sit and will flush out in morning when someone uses it.