How to create Dependent Picklist in Salesforce?

From the management settings for the object you want to add a field to, go to Fields. Custom task and event fields are accessible from the object management settings for Activities.
Click Field Dependencies.
Click New.
Choose a controlling field and dependent field.
Note
NOTE Some picklist and checkbox fields may not be available as controlling fields.
Click Continue.
Use the field dependency matrix to specify the dependent picklist values that are available when a user selects each controlling field value.
Optionally, click Preview to test your selections. If your organization uses record types, choose a record type to test how it affects your controlling and dependent picklist values. The record type controls what values are available in the controlling field. The record type and the controlling field together determine what values are available in the dependent picklist. For example, a dependent value is only available if it is available in the selected record type and the selected controlling value.
Note
NOTE The Filter by Record Type option doesn’t appear in the Preview window for activity custom fields.