For the info where to buy a valid SSL certificate please check this SO answer.
Process is described bellow:
- Step 1: Host with a dedicated IP address. In order to provide the best security, SSL certificates require your website to have its own dedicated IP address. ...
- Step 2: Buy a Certificate. ...
- Step 3: Activate the certificate. ...
- Step 4: Install the certificate. ...
- Step 5: Update your site to use HTTPS.