Most servers nowadays partner with Let's Encrypt to give you a free SSL. You can also use Cloudflare for another free SSL. Even though, it is recommended that you have a ready signed SSL in your server.
If your server doesn't give you one for free, purchase a cheap SSL, I prefer Comodo for starters (check Namecheap) and watch some videos or ask your server support to help you with that as it's not really easy process if you are doing it for the first time.