I know the answer but the site prevents me from answering because I have no reputation. What am I to do?
If you're logged out, you'll see a banner like this one in the place where the "answer question" box usually is:
Most of our questions don't have this banner and they can be answered by anyone. However, some of our questions attract low-quality answers from visitors who don't yet understand how our site works or what kinds of answers are valuable here. That particular question currently has two answers which have earned a positive score from the community, four answers that have a negative score, and (you may not be able to see this) two answers which have been deleted or hidden. Some of the downvoted and deleted questions were written by people who haven't had any other interaction with the site since. Apparently that question is not a great place for new users to begin interacting with our community.
To post your answer, you'll need to interact with the site in other ways (take the tour to see what this means) until you've earned a little reputation. One way to earn this reputation is to find an un-protected question which interests you, and answer it in a way that attracts an upvote from at least one other user.
When a question attracts a lot of poor answers, it can be marked as protected. This is done automatically in some cases and manually in others.
When a question is marked as protected, you need at least 10 reputation in order to answer it. So if you ask any question or provide any other answer that gets at least one upvote you will be able to answer protected questions.