Accessories for Nikon D5000?

I have a nikon D5000. I love it. I have the 18-55 lens and the 55-200 zoom lens. I don't have any accessories for my camera. I would like to get some filters and a hood and a external flash attachment. I don't really know much about any of that stuff. I'm curious as to what i need to get along with what each thing does.
what do filters do? What does a hood do?
and when to use them. Also when is a good time to use the flash?
any other accessories you recommend and why to use them and in what kind of situations.

1) Check Nikon's website and Amazon. 2) Since you don't *need* an external flash, you just want one, don't buy one yet. 3) There are many different types of filters - UV, Polarizing, Colored, etc etc etc. 3) Hoods protect the front of your lens and prevent/try to prevent glare.