Select the file/folder you want to share, then click on the "Share" button.
Select your audience, and choose who you want to share it with: with everybody within your organization, with some user(s), or with some team(s).
Just click on the "Confirm" button, and you're done!
Note: By default, when created/uploaded, files and folders will have restricted access: only Admins will be able to access them.