What camera equipment and/or accessories do I need to take pictures at a small event?

I have a Nikon D3000. I received it as a gift since I have always had an interest in photography. Some of my parents' friends ask me to occasionally take some pictures at events, nothing major but now, I was recently asked to be the official, and only, photographer at an event, bigger than I'm used to but still nothing major. I want to know what accessories are "must-haves" and which I could do without, that way I can buy only what I really need.

What you really need is a good dedicated external flash unit like a good used Nikon SB-600 and a flash diffuser. That and a hand full of freshly formatted 4 gb cards and batteries for the flash unit.

Perhaps the most important thing you need is experience using the flash, making sure that you set your camera to 1/200th second or slower All this is in your user manual