when ever i create a server my friend cannot join it, how do create a private server where we can play together?

If you mean to play in the same environment with your friend, you can play using one of 2 options as LAN or on this computer from the "party play" section.
you can use create server option and set maps and other features

If you mean in different environments and you want to play on the same server, the only option is online mode. If you add your friend to the buddy list, you can see the server you entered and play together.
(When you enter the game, you can click on your nick from the friend section on the right on the main screen and enter quickly)