SUM() & GROUP BY

hotel=# SELECT
hotel-# "public".billing.id,
hotel-# "public".billing.guest_id,
hotel-# "public".billing.trx_date,
hotel-# "public".billing.trx_time,
hotel-# "public".billing.payment_method,
hotel-# "public".billing.tax,
hotel-# "public".billing.dep_id,
hotel-# "public".department."name",
hotel-# SUM("public".items.price) AS total,
hotel-# "public".billing.amount_paid
hotel-# FROM
hotel-# "public".billing_items
hotel-# INNER JOIN "public".billing ON ("public".billing_items.billing_id = "public".billing.id)
hotel-# INNER JOIN "public".department ON ("public".billing.dep_id = "public".department.id)
hotel-# INNER JOIN "public".items ON ("public".billing_items.items_id = "public".items.id)
hotel-# GROUP BY "public".billing.id;
ERROR: Attribute billing.guest_id must be GROUPed or used in an aggregate function
hotel=#

What Worng ??

How to using SUM() & GROUP BY