Usually the way forward is to get one with your host and then it's a simple job to tick the "Use SSL/TLS" setting in Payment and Security and configure the network settings using https
As Jules said, the SSL certificate must be installed through your hosting (and there are many different ways that can happen - it depends on your host). You mention "SellerDeck control panel" - does that mean that you are using SellerDeck hosting?
Once that is installed, configure SellerDeck to use SSL as Jules says.
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