Welcome to Android Central! In the Samsung Contacts app, when you tap the + button, do you see a dialog box that says "Save contact to" and then gives you a choice between your Samsung account, Phone, and Google? If so, choose your preference there and tap Always.
If you don't see this dialog box, then you probably chose a default at some point and selected Always, but you just don't remember. In the Contacts app, tap Menu>Manage Contacts>Set Default Storage Location to change the default.