On Stack Overflow, I read that users with a reputation of at least 250 can vote to close their own questions. Where is this option, and how do I use it?
You should be seeing a "close" link below your questions, between "edit" and "delete".
Unlike deleting your own questions - which happens immediately - a vote to close is just that. Four other users must also vote to close. Actually, that's not strictly true - a diamond moderator's vote is final.