When I post a question and then find an answer for it (either other members have answered or I found the answer without any reply), how do I close the question?
If others have answered your question and you found them helpful, upvote them and accept the most helpful answer by clicking on the checkmark next to the question. I presume you're familiar with this, as your profile says 92% accept rate.
If you found the answer to your own question, post that as an answer and accept it (you might have to wait for 2 days before you can accept your own answer). That way, others who might have the same problem as you will find it helpful.
If you find that your question was really silly (like you forgot a comma in the code) and not likely to be helpful to anyone, and there are no answers and you've figured it out yourself, you can simply delete the question. You'll find the link under your question
link|edit|delete|flag ^ | this is what you need to click
You should mark one of the answers as Accepted by clicking the green checkmark next to that answer.
If you find the answer yourself, then submit your answer - there's nothing wrong with answering your own question!