I need to create a new view on a custom list that filter only the items that are added (created) by the users department, so any user can view all other items that are added by the other members of his department.
ok. sorry! I dont think that this is possbile out of the box. to get this working you would need to get and save the users department in a separate column on item create for example via an event receiver and after this you would need to create a "dynamic" view on this column.
use Target Audience in Sharepoint for this: