Hi,
To activate HTTPS on the checkout, you should turn on the option "Force SSL on checkout" in the Checkout tab of the configration.
Beware that you should already have a SSL certificate set up on your website when doing that. Otherwise it won't work.
To check if you have that SSL certificate set up and working just accessing your website with https instead of http in your browser URL bar. If that doesn't work, you should contact your hosting company in order to set that up (in most cases, it's not free) as it needs to be added directly on the server and we can't help you for that.
