New Laptop - Any Security Advice?

Hi team,

I bought a new laptop. Was wondering if anyone had any suggestions around things I should do to make sure my computer is secure. For example:

1. If I am sharing wifi with flatmates, is my computer or data at risk? Is there something I can do to protect myself?
2. Is there a way to ensure that I don't have a "public" drive available for anyone to view? A couple of years ago I discovered my flatmates drive was completely accessible to me. He had unintentionally shared it.
3. Any other ideas to protect my privacy or security?

My main concern is not with major companies, but with family members (especially my brother) from finding a way to look through my computer as he has done this before.