How to set up SSL connection between a client and server using C/Unix programming

I have written C program on unix. This is a client program and talking to a server program. It begins with Creating a socket, setting up socket parameters, connecting to server and after connection is established, then send message to server.
Now I want to set up SSL between client and server so that messages are encrypted. How to achieve that.
I know that on the server program, SSL certificate file need to installed. That is pre-requisite, I think. But how use that file to add SSL to the existing un-ecrypted connection, that what I am looking for.