"throw someone in at the deep end"

If you throw someone in at the deep end, you give them a difficult job to do, or a serious problem to deal with, before they have the knowledge or experience for it.

In her first overseas teaching job, Kathy was thrown in at the deep end with a class of 30 kids and no training, no books, and no guidelines.

Our manager thinks the best way to introduce new staff to the job is to throw them in at the deep end and see how they go.