Do I need a flash

Many weddings don't even allow photography and your flash might mess up the wedding photographer's shot. I suggest to everyone that they buy a flash, it's much better than the pop-up flash, but it's not very economical. The cheapest for your camera would be the SB600 at $224 from Amazon.

On the lens side, just use the lens you have and if you know about photography then you'll know what you'll need or don't need in the next lens. If you don't know much/anything about photography then take a course or buy a book and practice. It'll make much more of a difference in your shots.

There is no reason to feel scared. You should start playing with your camera and settings to get familiar with the controls. The external flash is critical for good flash photography. You should get one that can tilt/swivel to do bounce flash. You will also need a flash diffuser to soften the flash. The flash diffuser can be some cheap plastic that cover the flash or a bigger one like the Gary Fong Inverted dome light sphere. Bigger light diffuser soften the light more and give a more pleasing photo.

Since you don't know how to use your D-SLR yet, I will advise you to stay away from the professional photographer and let the pro get the shot before you snap one. The last thing the pro need is to worry about your unexpected harsh headon direct flash from your pop-up flash.

The best way to use a flash is to make the lighting look natural and balanced, taking away unwanted shadows, and people can't tell that you have used a flash. This is an art that requires a lot of practice and understanding how lighting works.

If you are new to photography and have little or no eperience with a camera, I suggest that you put the camera nto the auto mode and use it like a "point and shoot" to take pictures....and not worry about lens and flash. Do read the "quick quide" ..... and later,study the manual.

I suggest that you ask permission to take photos at the wedding, and try not to get in the way of others...there are "protocols"....

I would choose no flash in the guide mode or settings, as not to be distracting through the ceremony. Afterwords, you should be fine. I would definitely get some practice in before the big day. Otherwise, you may end up disturbing the sacred moment with explicit dialog from sheer frustration. It will take a little know-how to get what you want from these, more professional, cameras. Auto is good. I would stick with that but you still want to appear comfortable with your new camera, they are bigger and heavier that any point-and-shoot cameras out there. I would try to get a zoom lens, as well. Once I got my 55-200mm, which took forever, I was amazed at how much easier they can make it for you and how much clearer the results are. I'm not a giant fan of the 18-55mm kit lens. They take a little more patience to get really crisp shots out of.
Oh, and remember to turn the sound off as well. They can be loud. I would also look at many, many wedding images on-line so that you have an idea on style. Random shots without direction or a storyline tend to be less desirable and can appear unprofessional.
Try to shoot everyday. It is an ever-changing, renewing, challenging and fun learning process.

Good luck!