Scroll down to the bottom of the new thread page. You should see an option which is named "Post Poll" with an empty tick box. Check it, and select the maximum number of options you want in your poll. When you post the thread, it will not be posted, instead you will be directed to another page where you name the options in your poll and set a deadline for the poll to end if you want, among other things. After all that, you can save your changes and post the poll