Choose Home tab→Relate group→Ground button.
Select the part you want to ground. The part is grounded at its current location.
Tip:
The first part you place in an assembly is grounded automatically. To reposition the first part, you should first delete the ground relationship. You can then apply assembly relationships between the first part and the assembly reference planes or subsequent parts you place in the assembly.
You can also ground a part you are placing in an assembly by clearing the Maintain Relationships command on the Parts Library shortcut menu.