In order to be able to use freerelay, you need a certificate and a key. You will get the certificate from the freerelay after you have submitted your certificate request.
To create a new certificate, you run the command openssl req -new -nodes -newkey rsa:2048 -keyout key.pem -out req.pem. Then submit your request. Make sure you use a correct, valid email address in the request as you will get the certificate mailed to that address.