About the mandragora, or the mandrake plant and it's leaves.

The mandrake root and leaf is used in alot of transfiguartion potions, including the animagus process. It's mostly used in potions to return wizards who have been cursed or transformed against their will, back to normal.

The mandrake is a plant which's root is very human-like. At first it looks like a baby, but as the plant grows so does the root. It's cries are dangerous indeed. As it's stll small, in it's "baby phase", it's cries will only knock you out for a few hours maybe, so if you're handling the plant and planning on unearthing it, wear protective gear for your ears. But as it grows, and the root grows, it's cries also mature. Someone who's exposed to fully grown mandrake cries might die from them. They are fatal. 

Mediaclly, in ancient times especially in Europe, the plants we're used as painkillers or sedatives. 

You should learn about mandrakes in year two and three at Hogwarts, and a bit in year five too.

If you're planning on getting some mandrake leaves, the easiest is probably from the Herbology Professors greenhouse. Although if you should steal the leaves, or ask the professor I cannot say. What I don't know is how common mandrake leaves are, so if you ask your professor they might become suspicious and reject your request. You can always try though.

If you're planning on finding a mandrake out in the wild, and unearthening it, you should buy the protective gear from Mandrake Mufflers, a shop located on the north side of Diagon Alley.