Well when try to install a ssl certificate from comodo they send you a zip with 4 files.
AddTrustExternalCARoot.crt
COMODORSAAddTrustCA.crt
COMODORSADomainValidationSecureServerCA.crt
www_domain_com.crt
If you want to install this on apache it sholuld be something like this
SSLCertificateFile /path/www_domain_com.crt
SSLCertificateKeyFile /path/www_domain_com.key
SSLCACertificateFile /path/intermediate.crt
Where intermediate.crt is
cat COMODORSADomainValidationSecureServerCA.crt COMODORSAAddTrustCA.crt AddTrustExternalCARoot.crt > intermediate.crt