I have just tiled my shower floor with 6in. Porcelain tile. Should I use sanded or non-sanded grout? Should I seal the grout and/or tile?

Sanded and unsanded really depends on how large of a grout line you want and the look you want. Very narrow/small grout lines should not be sanded or you will never get the grout into the msall line. 1/4" or larger are almost always sanded grout, that size needs the body of a sanded grout.