Once it is clear what to do, everything gets easier. There is always the trap that you might be confidently taking the wrong path. The important thing is to stay alert and keep questioning your assumptions. If they hold, move forward. If they no longer hold, stop.
Now, if you genuinely do not know what to do, exploration is the best path. Let curiosity guide you. Run small experiments. Act. Sow seeds, and take feedback from others and from yourself. As soon as something starts to stick, give it more water. See if something is coming out, or could come out. If yes, keep giving it more time, invest more in it, and give less time to the rest. If no, keep exploring and sowing.
What should you do if you're young and ambitious but don't know what to work on? What you should not do is drift along passively, assuming the problem will solve itself. You need to take action. But there is no systematic procedure you can follow. When you read biographies of people who've done great work, it's remarkable how much luck is involved. They discover what to work on as a result of a chance meeting, or by reading a book they happen to pick up. So you need to make yourself a big target for luck, and the way to do that is to be curious. Try lots of things, meet lots of people, read lots of books, ask lots of questions.
Paul Graham in How to Do Great Work: