I assume you have lots of receipts where you paid cash or credit card.
The VAT rate on diesel is 21% (or 21.5% before 31 Dec 2009). If you have a receipt for €60, €49.59 is P&L expense, €10.41 is VAT reclaim and €60 is what you paid (bank, petty cash, credit card).
So you reclaim €10 in your VAT return under T2.
Your sales VAT goes into your T1 and T3 will show your payable / refund amount.
As the previous poster said, you should be recording your sales and purchases in some sort of record keeping system. Have you spoken to your accountant yet. Its important that you keep proper records as you go along to keep your year end accountancy costs down.
Hope that helps.