Go to the forum where you want to post the poll and click on the "New Thread" button. I suggest you try this in the Testing Area first.
Near the bottom of the page, you'll see a section for "Additional Options" such as attaching files and subscribing to the thread. At the bottom of the list is a checkbox for "Post a Poll" together with a field that allows you to specify the number of poll options.
When you submit the thread, you will be asked to enter the text for each poll option.
There's also another way for threads you already have created:
Click on "Thread Tools" in the green bar just below the "post Reply" button near the top of the page and select "Add a Poll to this Thread." The rest should be self-explanatory.