Hi Heriberto, I have never done this before but I just looked it up for you to help you out. It's in the FAQ section. I'll type it out for you what I just read;
To create a poll when you post a new thread, simply click the yes, post a poll with this thread checkbox at the bottom of the page and set the number of possible responses you want to include.
When you clickthe submit button, you will be taken to the Poll creation page, where you can specify the Question and the list of responses you want to include.
You may also want a time limit for the Poll so that (for eg) it stays open for voting for only a week.