If you want to explore more about this topic, read What is asset amortization?
Assets are a company's goods or rights. To book it, you must differentiate between the amount of its acquisition and the monthly amount that its amortization will represent.
The first amount is registered when the asset is invoiced for purchase, while the second one will be imputed as time goes by.
Keep in mind that, together with the liabilities and equity, the assets constitute the Balance Sheet.