Your TIN is technically your HKID number. I'm not sure how it works for etax (whether you get assigned a new number or not).
If you have never filed a tax return before, you will have to wait until you receive one. Typically this happen 18-24 months after registering your sole proprietership. If you don't receive the form, don't worry about it this financial year.
When you eventually do get the return, only file it for the year in question. The IRD will decide if they want you to back-file previous years of business operation (most likely yes, so keep all your documents).
Definitely call them if you are unsure. The english-language hotline is helpful (I might have said surprisingly but I'm sure the government wants every tax cent out of its populace).
I am someone who has started both a limited company and sole proprietorship, having never filed a tax return previously, within the last 6 years, so this is all a pretty recent experience for me.