Didn't you get a topup card included with your SIM/SIM+handset (which can be used in any store to topup your PAYG balance)? You may also be able to top via using your online account management, link: https://www.virginmobile.com/vm/login.do
as Shelke says, you should have a top-up card included with your Virgin SIM which you can use at any place displaying the green logo (lots of shops and garages). These same places will also sell vouchers which you can apply to your account.
If you've mislaid your card, you can request another one by calling the Team (free call 789 from your mobile) or by logging into your Virgin Mobile account.
As enlli has mentioned, many cash machines also offer mobile phone top-ups.