If you're in the mood, just sit down and write! It doesn't have to be great or anything - in honesty, my first draft sucked - but as long as you get the words out there, you can always come back and spruce them up. It is a fantastic feeling. This isn't so much as advice as a clarification that I'm sure many writers will agree with: a book is a lot like a puzzle. More often times than not, you won't start with chapter 1, then 2, then 3 and so on. You may begin telling your story by writing chapter 5, then work your way backwards and forwards. Writing is not a sequential thing.