@John Moorhouse it’s awesome that you are thinking of this at 16! I hope my kids get an early start like you. (My oldest is just 9 but if he’s asking this question at 16, I’ll be thrilled.)
I would say step 1 is simply to learn. If you’re into podcasts, check out ... the Bigger Pockets podcast! Seriously, I own a few rentals but just started listening and wish I’d started listening a while ago. I also recommend reading Rich Dad Poor Dad. That’s the book that started things off for me. Simply put, read, listen, learn. Learn the numbers and learn about property itself - construction, HVAC, electrical, plumbing, etc etc. You will never learn it all so get started.
Meanwhile, save up some money. It will take some time, but your first real investment will be your first home. You’ll want to “house hack”
It, as they say here at Bigger Pockets. And rent out a room or 2. Or if you can start with a multi-unit, Rent out the other units and let the tenants pay down your loan for you.
The advantage you have as a young, first-time home buyer is huge! You can typically buy with a lower downpayment (just avoid anything that is interest only or a variable rate - lock in that low rate for as long as you can) and of course you have a lot of years to live to pay down the loan.
This is all long term stuff but since you’re thinking this way now, I have a feeling you’ll be financially set at a much younger age than your peers. Good luck!