You don't need a keyboard for your Kindle Fire of course - it's a touch-screen device after all - but it might be handy if you send long e-mails or like to type up notes. A bluetooth keyboard will sync with your Kindle with ease - and because the "soft" on-screen keyboard doesn't pop up, you'll be able to see more of the screen. It'll make editing a lot easier.
There are a variety of different types to choose from. Some have the keyboard integrated into a case, others have a separate, free standing keyboard. Pick whichever style suits you best.
Here's a list of some of the best Kindle Fire keyboards available right now: