To get started, look for a link like "answer this", "improve this answer", or "edit this page". That will take you to a box where you can type a new answer or add to an existing one.
Once you're on the editing page, just type your answer and press save.
What do the icons do?
You can learn about these from the help pages, or you can simply type your answer and not worry about advanced options just yet!
Who can answer?
Anyone can ask questions and anyone can answer. You don't even need to make an account.
If you're logged in, you can be notified when your question was answered or your answer was improved.
You do not need to change Category:Un-answered questions to answered as these categories are automatic.
What is a good answer?
Try to give a good objective answer. Don't worry if your answer is not complete - anyone can edit the page again and add to or improve your answer. If the question has already been answered, try to edit the page to make it better. For that reason, it's best not to sign your answer.
What else can I do?
See Answering questions FAQ for more that you can do.