If you're a business and the goods or services you provide count as what's known as 'taxable supplies' (see 'What is VAT charged on' below) you'll have to register for VAT if either:
your turnover for the previous 12 months has gone over a specific limit - called the 'VAT threshold' which is £90,000 from 01/04/2024
you think your turnover will soon go over this limit
You can choose to register for VAT if you want, even if you don't have to.
For full VAT guide : http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?_nfpb=true&_pageLabel=pageVAT_ShowContent&id=HMCE_CL_001596&propertyType=document
Source of Information: http://www.hmrc.gov.uk/vat/start/introduction.htm
http://www.nationalarchives.gov.uk/doc/open-government-licence/