You can make a reservation at https://ferry.ncdot.gov/
It can be crowded at times, so I recommend making a reservation at least one day in advance. I have had to wait a few times.
Many people try to take a shortcut through the Camp Lejeune military base as they approach the Cedar Island ferry from the south on NC 172. This road is shown as a public road on most road maps, but I think it is no longer open to the public. Last time I tried to go through, I was not allowed. I suggest you should stay on US 17 north to NC 24. Though, if you have a day or two, you could continue north on US 17 to New Bern, which has some interesting history. (the first capitol of NC, second oldest town in the US)