Temporary (will be deleted after the last user left the channel).
Click on the Servername and then Create Channel. Now you can tap in your Channelname etc.
In the end click on OK.
Your channel is created.

Permanently
please do the same things like the temporarychannel.
After you did this please contact a teammember.
The permanentlychannel will be deleted after two weeks if they are empty.