Rules on grounding on a PCB with low voltage digital circuit
Ground policy is not something you can learn in a single post. You must understand, that it is affected by the board architecture, the system, the enclosure, the standards you have to comply with. 

Still, the question itself suggests that you are not going to pass EMI tests, so here are some simple guidelines :

Make a good ground plane- wide, as few holes as possible, thick copper layes. 

Where you have high currents, cut the ground plane so the current is routed directly to the power source (connector? Filter) and does not flow under digital or analog circuits. You can even define DGND, PGND, etc. and connect them in a single point with 0R resistors. 

Put capacitors that will connect AC to metal enclosure. Usually it's like 1uF and 10nF near each mounting hole. This is effective because the enclosure is thick and its impedance is very low.

For analog circuits pay attention that no digital return current flows under them. 

Most importantly, have second chance to fix any problem with new layout.