Follow the tutorial How to Configure an SSL Certificate for Apache on Amazon EC2 on splittingelectrons. Also consult the relevant RapidSSL Technical Support articles RapidSSL – Generate Certificate Signing Request (CSR) and RapidSSL – Install SSL Certificate.
* Since the splittingelectrons blog has not been updated in 2 years, I’ve copied its contents into an HTML comment embedded in this post so it doesn’t get lost.
[UPDATE Monday, December 1, 2014]
Beginning next summer (2015), you should look into Let’s Encrypt as your *free* certificate authority!
See “Launching in 2015: A Certificate Authority to Encrypt the Entire Web” by Peter Eckersley on EFF.org, November 18, 2014