First click Teams 
on the left side of the app,
click Join or create a team 
at the bottom of your teams list.
Then click Create a team 
at the top right of the app.
Choose Build a team from scratch.
Next, you’ll want to choose what kind team you want this to be:
To limit content and conversation to a specific set of people, choose Private.
For a community or topic that anyone in the organization can join, choose Public.