I'd recommend taking a soil sample and getting it tested before adding fertilizers.

If you want to go organic, I've posted an article called "How to Make Organic Garden Soil".

If you just want to bring up the nitrogen, buy some lawn fertilizer and put that down.

When Should I Fertilize My Lawn?

Be careful the article on fertilizing your lawn states that too much nitrogen at the wrong time will just encourage weed growth. There's a chart on when to fertilize and what ratio to use. I think you might have time to add some fertilize before December.

Here is a link that might be useful: How to Make Organic Garden Soil