How can I create my own public certificate using openssl?

I have a public key/private key pair i generated using two primes p and q. I now need to use openssl to sign and create the certificate. I heard req utility and wrote this from the man pages.

openssl req -x509 -days 365 -nodes -key inputfile.txt -keyout key.pem -out cert.pem

But what details should I specify in inputfile.txt and is there a particular format for that?