18-55mm is good enough for wedding shots. You definitely need an external flash. The built in flash is no good 
Nikon SB-600 is a good flash.
I would also recommend getting some book/online tutorial on understanding exposure (aperture/shutter combo) and read through your camera manual so you can use modes other than Auto 
If you decide to go with a non-Nikon lens, I would recommend a Tamron lens over Sigma.