I started a thread and then I realised that the poll would be a good idea. What I can do?

If you made the thread then you can click on thread tools & edit it.

If you didn't make it then you'll need to get a mod to do it for you.

I tried but I didn't found any way to do the poll

New Thread, check the Add a Poll box, fill the number of options (2 to 10), post, choose the options of the poll then that's it !

look in the same place that you used to post the thread cept when the thread is open and click add poll