Backgrounds

I have a question for you all, I want to take some pictures of my little boy how do I get a black background????
I no how to do it in pp but using lr3 at minute

The obvious would be to hang a black piece of material behind him.
You might also try some scenes with a really low depth of field (low F number). This would blur all the background nicely while having a crisp focus on your son.

Go to an upholstery supply warehouse and buy a few yards of black vinyl...you can find them online too...