To create a tag, one must apply it to an existing question. Any user with the Create Tags privilege (reputation required to earn it is site specific) may add new tags to any question at will by typing the new tag into the question's tags field, and submitting the post or edit1.
If you have enough reputation to create the tag, and if you think there is a clear need for a tag, go ahead and create it yourself. If the new tag causes controversy, you can always come back to your site's meta to ask the community to judge it.
If you do not have enough reputation to create the tag, then add a comment on your question. Say that you wanted to add the tag to the question, but do not have enough reputation. Please consider including a reason why you think this tag is appropriate. If someone with enough reputation agrees with you, they will be able to create the tag. Please delete your comment once this has happened. As an alternative, you can request creating a tag by starting a new meta discussion. This gives other people the opportunity to discuss if a new tag is needed.
1 Exception: a small number of tags may have been blacklisted on your site because they're not suitable; these tags cannot be recreated.