I would like delete a question of mine since it's been closed. Is there a way to delete a closed question? I am owner, so why am I not able to delete it the normal way? Do I have to ask some moderator?
After 24 hours you can vote to delete your question. Other users with over 10k rep can also vote. It takes 3 votes to delete a question that is closed (it also takes 3 votes to reopen).
Or a moderator can do it on their own. If you are that intent on deleting it, just flag for mod attention and someone can make the call.
For more information: Do Closed Questions Ever Get Deleted?
Note: It should also be noted that even though you are the primary owner of your questions, this is a CC-Wiki site and all content is inherently owned by the entire community. Just because you may want your question deleted does not mean that others won't want it to remain so that it can help out users in the future.
If you are still able to flag the question, I would flag it for a moderator and explain you'd like it deleted. Otherwise, I would flag another one of your posts for a moderator and explain that you'd like it deleted, with a link to the question, and why. In time I would expect them to take care of it, unless they believe there is reason for it to stay around. While in some cases it may seem clear that a question can and should be entirely removed, in many cases there are reasons to keep closed questions around, and therefore, non moderators cannot delete questions on their own.