I always use a spoonful of clothes washing powder in the teapot. Fill with hot water then leave to soak until the stains come away. Make sure you rinse the teapot thoroughly before using again!
Assuming it was only used for drinks, it's not too tough to clean. Put in a very, very small drop of dishwashing soap, followed by hot water (about half way full.) Close, shake, vigorously, them...
Try a product called Bar Keepers Friend. It comes in a liquid or a powder. You will need a bit of elbow grease as well, however, that is sold separately.