You need to follow the 30 day's free-trial instruction.

Which HTTP server you are using, Apache ?

You can follow the 30 day's free trial instruction. but first you have to generate your server's CSR first and copy and paste to the free trial page:

After generating your server's Certificate Signing Request as described in Generate CSR, paste the CSR in the form below. Please make sure that it contains the complete header and footer "BEGIN" and "END" lines exactly as in the example below.