I already have purchased SSL certificate and i have received certificate and a .pem file as a private key? from the supplier; now i need to convert this .pem key into .key for b
If you're looking for a file to use in httpd-ssl.conf as a value for SSLCertificateKeyFile, a PEM file should work just fine.
See this SO question/answer for more details on the SSL options in that file.
Why is SSLCertificateKeyFile needed for Apache?