This is often the forgotten tax! The threshold for registration is currently £85,000 of taxable supplies over a rolling 12 month period. Many businesses may start off not being VAT registered however it is very important to keep monthly records of turnover to ensure you do not breach the threshold. Breaching the threshold will require you to register for VAT and then normally complete quarterly VAT Returns.
It is not always best to just wait to breach the threshold before registering. If your customers are all or mostly VAT registered, you may be better off becoming VAT registered in order to recover the VAT you suffer on purchases but not also becoming more expensive to your customers. Another reason could be because you are a repayment trader; the most common examples being farmers or those making sales to an EU country.