be thrown in the deep end
To be prompted or forced to begin doing something very complex and/or unfamiliar, especially suddenly and without guidance, assistance, or preparation. I had never worked in sales before, I was just thrown in the deep end! We were thrown into the deep end the first day of class when the lecturer asked us to write a 2,000-word essay on one of Shakespeare's plays. Those who go the furthest in life are the ones willing to be thrown in at the deep end when a great opportunity arises.