I had a quick search and found how to install an ssl certificate on digital ocean
. If thats not exactly what you want theres more information available on the digital ocean website.
As far as phpBB is concerned, once the certificate is installed, all you have to do is enable cookie secure. See Fixing incorrect cookie settings
. You may want to put a redirect (http to https) in your .htaccess file to stop anyone connecting to your board with http.