If using a sb800 just set it to ttlbl on the flash gun and then it will just fill it. If its anything like the d300 youll need it on matrix or centre weighted for ttlbl to work.
Then either shoot in manual and the camera and flash will work it out-or shoot in ap and it will do the same. Dont forget if its a bright day youll need the flash sync speed set to fp or it will give you a max synv speed of 1 250th and then if it requires a shutter speed of say 1 500th it will not be fast enough and overexpose it by a stop or 2.
Hope this makes sense and helps