A good essay is more about you than you might think. It certainly isn't about just writing what you're told (although the HSC can make it feel like that from time to time). No, it's about developing and proving ideas, showing that you know what you're talking about, and having a logical, clear way to make your argument. That's why a thesis is so important. It creates the path for your reader. It says, "'ere's wot I'm talkin' about" which implies that your thesis has a cockney accent or that I made a few typos and just ran with it. A great thesis catches attention and sets the direction.
Your thesis needs to demonstrate that YOU have started to become a thinking member of the human race. No pressure though! A poor thesis screams, "I'm not ready" because it's too simple. It doesn't explore an idea or even imply that an idea needs to be explored. They look like this:
Journeys are important because we all experience them in our own ways.
Each person's discovery is interpreted differently and all discoverys are valid.
Even if these statements were true, they're still boring. They don't say anything that's worth exploring. They also don't say anything that can be proven. To prove "we all experience them in our own ways" you have to give evidence of all the different ways. Good luck with that. Recognising that each person interprets discoveries differently means very little because it's also too broad, and because it simplifies the idea too much. So avoid such things.
DON'T simplify the idea too much.
DON'T try to come up with a broad phrase that summarises the entire concept.
DON'T explore boring ideas. Demonstrate that you've started thinking about great ideas and what they mean.
NEVER FORGET THE QUESTION!