Let's Encrypt (Free SSL Certificates) should be enabled automatically by default. However if it isn't, there is a way to enable it manually from cPanel.
Step 1 - From cPanel, select SSL/TLS Status
Step 2 - Make sure the boxes are ticked for the domains/subdomains you want a SSL for and click Run AutoSSL
Note 1: You may have to wait 1hr for the free SSL to apply
Note 2: If under the list of domains, the chosen domain/subdomain has + Include during AutoSSL under the Certificate Status, available, make sure to click that first.
If you have any problems, click here for our guide on fixing autossl problems.