You will need to first generate a Certificate Signing Request (CSR) and then use it to order an SSL Certificate from a provider and install it. The provider you would buy your certificate from should be able to explain you how to generate a CSR.
Hope it helps.
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As stated previously, in order to use SSL for a website, you need to get a SSL certificate. Generally, you need to purchase these; however, you can also generate your own if need-be. Although, if you generate your own and visitor to your site will get a warning about this.