Jump/be thrown in at the deep end
Definition 1 of 1
Pro Tip 1/3Sudden Challenge
Used when someone faces an unexpected difficult task without help or prior experience. 
Despite being new, Lisa was thrown in at the deep end to negotiate with clients.
Pro Tip 2/3No Gradual Introduction
Emphasizes the absence of a step-by-step learning process or gradual increase in difficulty. 
I had to jump in at the deep end with my new software project.
Pro Tip 3/3Emotional Overwhelm
Can imply feeling overwhelmed due to the sudden increase in responsibility or difficulty. 
I felt thrown in at the deep end when I had to manage the event alone.