Choosing an SSL mode defines the https connections between your visitors, Cloudflare, and your server. Flexible can be used regardless of whether you already own a certificate, Full should be used if you have a self-signed certificate, and Full(Strict) is for users who own a certificate signed by a valid CA. For a full description of the various SSL modes, see What do the SSL modes mean?.
If you want to use HTTPS only on your website, we have a guide that explains how to redirect all visitors to HTTPS.
We also provide other options in terms of SSL, including the ability to upload a custom SSL certificate (business plan feature) and Keyless SSL (for Enterprise customers).