Do I need a splash shield to put my fender on
 

I just bought a 1991 300z and It didn't have a fender on it the car , if I order a new fender will it fit properly with out the splash guard under the fender

The splash guard is plastic. The fender bolts (attaches) to the body... so no, you don't "need" it to install the fender. But I do suggest installing it to protect your fender and other items near the wheel well.