jump in the deep end
To begin doing something very complex and/or unfamiliar, especially suddenly and without guidance, assistance, or preparation. A: "But I've never worked in sales before!" B: "You'll be fine, just jump in the deep end and try your best!" I'm a little nervous about starting my graduate degree program, but I'm determined to jump into the deep end and give it a go. Those who go the furthest in life are the ones willing to jump in at the deep end when a great opportunity arises.