I have just been helping a friend order a Birth Certificate from the UK online. Once you know how its a straight forward process.
I recommend that you first find the birth, death or marriage details at FreeBMD-Search. Make a note when looking at the record for the name of the District, the Vol and the page number as you will need this information when placing your order for the certificate.
Here is the link for the FreeBMD-Search
This link will take you to the site to order your Birth, Death or marriage certificates online.