How do I write a motivation letter as a school leaver?

Short answer: One page, in your own words, specific to the course. You do not need big achievements. You need a clear reason and real examples.

Why this subject: one moment or project that made you choose it.
What you have done: HSC subjects you did well in, a science fair, a club, a coding project, volunteering.
Why this course and university: name one or two modules.
Why Italy: concrete reasons, not "Italy is beautiful".
Your plan: what you want to do after the degree.

Reference letters from teachers: ask teachers who know you, on school letterhead, signed, with contact details. Never write, sign or upload a letter in a teacher's name. That is fraud.

Keep the same story everywhere. The embassy also checks whether the course fits your school background. Your letter and your visa answers should match.

This page explains official rules in plain language. I am not an immigration adviser and I do not give visa advice. For that, always use official government sources. Rules change: confirm with your university, the embassy and VFS before you act.