Photoshop copy section and fill it with patter from image

Hey there I am somewhat new to photoshop and here is what I am trying to do. I have a picture like this

Now I have an area that I have selected and what I want to do is set this to part of the out of focus background. So I want to copy a section of the background that is the same size as my selected area, or the whole blank area and set it to the background.

How can I go about doing this?

Thanks for the help in advance.