throw (one) in(to) the deep end

To prompt or force one to begin doing something very complex or unfamiliar, especially suddenly and without guidance, assistance, or preparation. I had never worked in sales before, but my boss just threw me in the deep end and expected me to learn as I went. The lecturer likes to throw his students into the deep end on the first day of class by having them write a 2,000-word essay on one of Shakespeare's plays.