1) Purchase SSL from link below and M3 will install it.
FREE SSL with positive Facebook: Leave Facebook Review
2) Login to wp-admin
3) Click Settings
4) Click General
5) Change both urls from http to https and click save.
If website still does not have green lock then use the blog below to find mixed content.
You can also temporarily install plugin like Better Search Replace. This will change the remaining mixed content from http to https.
Force https is done though .htaccess. Below link will give exact templates to use just change domain name to yours.