Is their any campsites near the river? Do I need to reserve?

If you go down river, towards engelbright there are some boat sites, but there aren't any campsites in the State Park.

What is the safest best campground near the south fork?

You can park at Bridgeport and take the trail downriver to Engkebright Lake. Great sites(about 10). I think hike is a few miles. Campground is at the joining of both forks of the yuba. One fork is warm and the other ice cold!