Before you can create tags, you must create Tag Groups to contain them.
To create a new tag group, go to Settings → Tags and click New Tag Group.
Each tag group holds a unique set of tags, and lets you define a custom set of fields that should be available to tags within the group. However, you don't need to assign any fields to the Tag Group Field Layout in order to use the group.
To assign tags to things (like Entries), you must create a Tags field and add it to a Field Layout.
Each Tags field is connected to a single tag group. Whatever you attach the field to (entries, assets, users, etc.) will be able to create new tags and create relations to any of the tags within that group.