How to change Domain name

Hello everyone!
I am using a Windows 2003 and I want to connect the 3 computers to the server. I have the IP address, I configure already on the server as well as the 3 computers. When I tried connecting PC1 to the server, it doesn't work? Upon checking the manager properties of the computer, I noticed that the computer name set to Workgroup and it should've been in domain.
How can I change it to domain instead of workgroup?
Do I have to uninstall and install Windows 2003 again to be able to connect to the PC?
I need your opinion thank you.
