How To Create Calculate Field On 3 Table Using Group By

I am a student, and I am so confused by SQL code that calculates incomes of my contract in a year or each month of year.

I have 3 tables:

lodgings_Contract:
id_contract indentity primary,
id_person int,
id_room varchar(4),
day_begin datetime,
day_end datetime,
day_register datetime
money_per_month money

electric:
id_electric indentity primary key,
id_room varchar(4),
number_first int,
number_last int,
Sum_Number int,
money_electric money,
status bit

Water:
id_Water indentity primary key,
id_room varchar(4),
number_first int,
number_last int,
Sum_Number int,
money_water money,
status bit

Now what I want to do are statistics on how much money I got in a year or month. Here is my code to calculate incomes of year.

Select Year(day_register) as 'Year'
, Sum(money_per_month * month(day_end-day_register)) + sum(b.money_electric+c.money_water) as 'Incomes'
From lodgings_Contract a
, electric b
, Water c
Where a.id_room = b.id_room
And a.id_room = c.id_room
And b.status = 1
And c.status = 1
Group by Year(day_register)