There are 4 day books (ledgers). Sales, purchases, bank and nominal. the sales ledger records all your invoices and associated credit notes. You calculate your sales and VAT output figure from it. The purchase book is for purchases and VAT. Bank for all bank transactions (receipts and payments and for reconciling it). the nominal is for any adjustments accruals/prepayments, bad debts provision, etc.
Get a free package from TAS/SAGE - the day of writing these things up is gone esp. with free software (unless of course you only have a few transactions and you have no access to a computer).