I take it you are self-employed. The straight answer to your question is you must register when your tax supplies (to your customers) exceed �64k on the preceeding 12 months basis. You can however elect to register for VAT if your supplies are less than this. The website link below tells you more about it and how to appply (which can be done on-line).
http://customs.hmrc.gov.uk/channelsPortalWebAp p/channelsPortalWebApp.portal?_nfpb=true&_page Label=pageVAT_InfoGuides&propertyType=document &id=HMCE_CL_001222#P22_1118