As I have noticed we new covering certain areas. If you think we need a perticular tag send it in with a description so it can be created after up voting. Limits one tag proposal per entry so voting can be done.
Tags don't have an independent existence. Questions have tags. If you ask a question, or see a question, which would benefit from the addition of a tag which doesn't currently exist, then add it*. You don't create the tags first.
* If you don't have sufficient rep to add a new tag, then it might be appropriate to post on meta, with a link to your question, asking someone else to add the new tag for you.