As you say you don't get any miles is looks as though you have a Virgin MONEY card.
To earn miles you need either the white or black Virgin Atlantic MBNA Amex cards. This isn;t the same as the Virgin Money card and is offered by a different company. But there is an annual fee for the black one. But there are some sign up bonuses of you spend £X in Ymonths. However nice the bonus miles are you need to check to see if the cards are right for you and there are changes ahead with AMEX ceasing to allow 3rd party providers from offering AMEX cards.
Other ways to earn miles is to sign up to answer e-rewards surveys. Ther are also other VS partners listed on the VS website.
As to how many miles you need - just do a dummy booking on the VS site (selecting 'use miles') and it will tell you. Though remember availabilty is always limited.