It is used by secure web servers for shopping malls, telebanking and things like that. SSLeay is a free implementation of the SSL protocol. The current version while these lines are written is 0.9.0.
Encryption is plagued by legal problems of all kinds.
Get and install SSLeay. Modify the Makefile. At the end of the configurable section you can find the SSL definitions. Compile and install as usual. Before you configure SSL, you should know the basics of cryptography and how SSL relates to all of this.