You can go to the website of the Association of Private Client Investment Managers and Stockbrokers (APCIMS) and look at the page displayed 'How to Buy and Sell Shares' (you may also want to explore other parts of the site first, such as the company history). You can then click the tab that says 'find a broker or wealth manager now' and get in touch with a stockbroker. He or she will talk to you about how many shares you want, how to pay for them and any other questions you have.

however, a good stockbroker may warn you that now is perhaps not the best time to buy shares in Facebook. After all the excitement when it was floated, shares in the company haven't been doing so well. The most up-to-date information I can find says that the shares have already fallen by 11%. Before contacting a stockbroker you may want to do some research into how well the shares are performing.