Being a freelancer, you are forced to look into the other details of your work such as managing your money yourself. Most agencies would want you to submit an invoice that will give descriptions to enable them to make the payment that is due to you. But, if you are a newbie to freelancing, you might not know what details you need to include in the invoice to enable trouble free payouts.
Though there is no specified format to the invoice, every agency that is asking for it would like you to put in some details for reference. You can also refer to the payment terms and conditions that were discussed with you at the time of providing you the work for useful information such as tax liabilities, special clauses if any, etc that will be useful while generating the invoice.
Having all the background information ready is the first step to successfully readying up the invoice. You will need to include relevant information like your complete name, address, the price agreed upon, VAT number, the payment details that were agreed upon (such as Bank account, paypal or moneybookers, etc) and the VAT amount that needs to be paid, etc.
A few details about the project too need to be included to enable the agency to know which project the payment is due for. This can have a column which provides the title of the project and another two columns that include details such as start date and end date. It can also include the dates that had been determined as deadline for the completion of the project. All these details will not only help as a reference for payment but will also help at a later date for reference.
If you have difficulty in generating an invoice by yourself, you can even create an invoice by using some software such as Practicount and invoice, TO 3000, etc. There is also some freeware available on the internet that will help you in generating the invoice.