To register your expenses, you need to create records in "Invoicing -> Manage invoices (Purchases) -> Add". There, you can enter your expenses explicitly (as a product/service) or by putting a generic title (eg office expenses) and proceed with the payment, just like you would do with sales documents.
Alternatively, you can simply create direct payments through the "Cashflow -> Payments Management menu -> Add direct payment". The difference between this and the first option is that these records are not involved in the contacts' statements or the purchases reports. So you cannot keep track of the balances or include them in aggregated reports for expenses.
We recommend you to choose the first process, as it offers more options.