This is often the forgotten tax for small businesses! The threshold for VAT 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 know when you breach the threshold.
Breaching the threshold will require you to register for VAT and become Making Tax Digital compliant (MTDfV). A blog post can be found here. Your business will normally complete quarterly VAT Returns, although there are other options.