Hi,
To tag a document in Share navigate to "Document Library" select a document which should take you to the - "document details" page.
Click on "Edit Tags" under the Property Sheet [on right hand side].
This should take you the "Edit Properties" page for that document.
Click on "Select" under Tags section which brings up a new window where you can add new tags or you will have the option to select already existing tags.
If you need to manage your tags [view/edit] the other option is to navigate to "Admin Console" > "Tag Manager" .
Thanks,