Basic HTTPS Question

I am wondering how to set up HTTPS for my website. When I use https:// it says my website's not secure. Someone I need to use letencrypt or pay for a service. I am wondering if anyone can walk me threw how to set up HTTPS on my website through cloudflare. I also want it to effect my subdomains as well. I am running a cloudflare tunnel as well so I don't know if that effects anything...