Hello, i have created a server from my root and have created a user for it. After login into that user, i have created a sub-server for a subdomain. Now how do i create a user to have access and control over the sub server 
If you created a sub-server, you’ll use the administrative user of the parent server to access it. That’s what sub-servers do/are: They share the admin user and the filesystem path with the parent.
yes, but i want to provide my friend with his own control panel, and he could only manage that particular files
Howdy,
To provide your friend with his own panel, my suggestion would be to change that Sub-Server to be a top-level Virtual Server.
Then, your friend will have access to just that one domain.
You can do that by going into Server Configuration -> Move Virtual Server, and there you can convert it to a parent server.
-Eric