Hi Beatriz and welcome to the Disney Parks Moms Panel! This is the type of question where I could go on and on! However, we do have limited space we can use for our answers so please, feel free to send in as many questions as you need to as you plan your vacation!
As far as when to go, between the months you've selected, I prefer the November time frame. I have always found November to have perfect weather with very little rain, and the crowds are generally lower (with the exception of Thanksgiving weekend). However, I have vacationed in September many times as well and I'll be doing it again this year so it can't be that bad either! :) But it is warmer and you will see more rain.
My suggestion would be to keep an eye on the
special offers page. If you see an offer that suits your budget let that be your guide. For example, Disney is offering free dining for most days this September. I'm not sure if that is something they will do next year but it's a great example of what can be offered in September. As I look at the special offers page further, I notice another free dining offer which includes the November time period for 2013 as well as a discount code.
Since you are planning your first vacation, you might find the
Disney Vacation Planning DVD and the
Planning Your First Trip guide helpful.
We look forward to hearing form you again, Beatriz!