Now how to use sub-domain?

Have made a custom domain but now the problem is couldn't manage it.I don't know how to use it.I have tried several times but fails all the time.From last three days I am trying.Oh there is always any problem to handle my new blog.
I have create my mail account through Godaddy which they offered free with hosting package.The mail account is with my domain,they have offer me another free mail from hosting.But now main problem is how to use sub-domain.I have to search on Google.See what can I get from their.Thanks.