Little help on setting up SSL on my domain

I got an ssl certificate from PositiveSSL but problem is i don't know how to set it on my domain.

Within email i received 3 files called

AddTrustExternalCARoot.crt
PositiveSSLCA2.crt
www_mydomain_tld.crt

But files in mydomain.tld/ssl folder are a bit different. Currently i have self signed certificate generated from ispconfig control panel and files in ssl folder are these:

mydomain.tld.crt
mydomain.tld.csr
mydomain.tld.key
mydomain.tld.key.org

So i don't know what is what and where to copy what files.