Just to expand on Pete's correct answer,
Create a payments table, make sure you store the PaymentDate and PaymentValue fields + whatever else you require (who made the payment, payment source, short description etc)
Something like (SQL Server Syntax)
CREATE TABLE [Payments] (
[Id] [int] IDENTITY (1, 1) NOT NULL ,
[PaymentDate] [datetime] NULL ,
[PaymentValue] [money] NULL ,
[Description] [varchar] (50),
[PaidBy] [varchar] (50)
CONSTRAINT [PK_Payments] PRIMARY KEY CLUSTERED
(
[Id]
) WITH FILLFACTOR = 90 ON [PRIMARY]
) ON [PRIMARY]
GO
This will contain individual payments you receive. You can then write a query that summarises data by month