To the right of your question there is a lot of buttons. There should be a call a vote button. Press It. There you go! You have to wait at least 24 hours before you can do this.
To be sure we're all on the same page, we're talking about _putting a question up_ for vote. (There's no way to actually turn a question _into_ a vote!) Lara_croft and Jane_Furrows are both right as to how this is done.
Go to your question page.
If you don't know where it is, you can find it by clicking on the My Q&A at the top of the screen or by doing an advanced search for your user name.
To the right side of your question will be the option to call a vote.
Note: You have to wait 24 hours after you ask the question to call a vote. You can only call a vote if your question has received two or more responses.