your site is
www.buygeminixxx.com .
Just go thru the process, and see , when you get to the credit card data entering page, that you are still on
http://www.buygeminixxx.com/whatever/.
Then you need a ssl for
www.buygeminixxx.com
If you do go to
http://buygeminixxx.com, then you need a ssl for buygeminixxx.com. In this case, in your config file of OsCommerce, you must indicate that your site is for http "buygeminixxx.com" and for https "buygeminixxx.com"
As for the certificate, use Cpanel for your domain. Generate a private key, and save it for future use.
Generate a Crt request. Use that in your ssl provider to obtain a certificate. They will send you the CRT and Cabundle .
In Cpanel, enter the private key, crt and cabundle. Click done.
That it is.