Click the + in the inbox header and pick Channel. Give it a name (lowercase letters, numbers and hyphens — e.g. #late-shift-handover), an optional description, and pick whether it's public or private.
Public vs private
Public — appears in the channel directory; anyone in the organisation can find and join it.
Private — invite-only. Good for safeguarding, leadership or sensitive topics.
Initial members
You can add initial members straight from the create dialog, or add them later from the channel's members panel.
You're the owner
Whoever creates a channel becomes its first owner — they can rename it, change the topic, add or remove members, change roles and archive it. Owners can promote others to share that responsibility.