jump in/be thrown in at the deep end
jump (or be thrown) in at the deep endface a difficult problem or undertaking with little experience of it. informal
jump in/be thrown in at the ˈdeep end(informal) try to do something difficult without help when you are not prepared or know very little about it: On the first day of her new teaching job, she was thrown in at the deep end and was told to teach the most badly-behaved class. ♢ I didn’t know anything about business when I started. I just had to jump in at the deep end.
This phrase refers to the deep end of a swimming pool, where it is too deep to stand.