Sink or swim. Succeed alone or fail.
Thrown in the deep end. Given a difficult task with little preparation.
Trial by fire. A first test under real pressure.
Learn the ropes. Learn how things work by doing them.
Figure it out. Find the answer without being told.
Self-starter. A person who begins work without waiting for instructions.
Wear many hats. Do several jobs at once, often in a small company.