I was writing a question for which none of the currently created tags would be representative and I had no reputation enough to create a new tag. What would be the correct way to proceed in such a situation?
There are three ways to get a tag added to the site:
The easiest way is to suggest an edit, i.e. click the edit or improve this question button (so named on some sites) below the question. It would be peer reviewed and approved if it's good. A positive side effect is, that you would get +2 reputation for an accepted edit suggestion, which brings you nearer to the right to create tags and edit questions and answers.
The other acceptable way would be to flag the post for moderator attention. This works for any suggestion or problem with a question or an answer, so also for tagging suggestions. Don't hesitate with flagging, better point to something than leaving it unsolved. Moderators are there for it. If you would be unsure if a tag would be a good choice or not, or regarding a similar general question, the meta site is a good place for discussing it together.
Another possible way would be to comment to your own question, mentioning this, so somebody could add the tag for you. But this would require to remove the comment afterwards. Using the suggested edit feature is a better mechanism. I mentioned that way because we don't have our own moderators yet, but I'm sure the SE moderators would jump in. So better let's go back to point 2.