how to connect my computer to a domain?

Hi.

I can't to connect my compuer to a domain.

I checked the pinging and i got a reply to the windows server .

After i'm getting to My Computer, and then click Properties. The System Properties .

I click on " change" and when i'm trying to click on the " Domain" i can't becouse it's closed.

I can only choose the workgroup.

How can i open the choose "Domain"?

Thanks

Eliran Onel