I'll assume you already know about tone and that sort of thing.
Structure-wise...
Business letters often use a letterhead. They'll also have the date and address of sender in the top right corner. The address of the recipient is a few lines down on the left hand side. Below that will be a subject line (Re: Your New Bank Account or somesuch) and below that is the recipient's name.
Body of the text is usually in blocks with a few lines between each block for clarity. They may or may not be indented, but I doubt that bothers you
The signoff is formal (Sincerely, Thank you for your cooperation, etc) and there is often a signature above the sender's name.
A personal letter more likely has just the date up the top (but if you want to score marks put the address by all means) and leaves out the recipient's address. Paragraphing is usually a lot tighter (this doesn't matter in the HSC of course) and it's usually a lot longer than a business letter.
I think that's right and I hope it helps! It's easy to find information about personal letters online. Look at bank letters or your parents' business letters for more examples. As with everything these days there's no set standard for writing either business or personal letters.