DNS settings:
Make sure your domain's DNS records are set up correctly in your Cloudflare dashboard. The A, CNAME, MX, etc. records should be correctly pointed to your server.
Use the "Proxy" option (orange cloud) for A/CNAME records that you want to protect and optimize with Cloudflare.
SSL/TLS:
Make sure your SSL configuration is correct. Depending on your server settings, you can select either "Full" or "Full (strict)" in your Cloudflare SSL/TLS settings. It is important that an SSL certificate is installed on your server.
Enable "Always Use HTTPS" and "Automatic HTTPS Rewrites" if you want traffic to be automatically redirected to HTTPS.