Step 1: install a certificate
Most hosts offer free Let's Encrypt certificates. Once installed, your site serves over https and the warning disappears.
You rarely need to pay — Let's Encrypt certificates are free and most hosts install them in a click.
Step 2: force https everywhere
Redirect all http traffic to https so nobody lands on the insecure version. Then fix any mixed-content links (images, fonts, scripts still on http).
Step 3: keep it renewed
Free certificates auto-renew, but only if configured. Set a reminder or use a host that renews for you, and monitor the expiry date.
Key takeaway
Install, force https everywhere, then keep it renewed — that's a padlock that stays put.