How to get a VAT exemption cerificate?

We are new VAT registered business selling medical instrument, is there any way to get a VAT exemption certificate?

There is no such thing as a "VAT exemption certificate". Who is advising you on this? If you mean a zero rating certificate for certain medical equipment, these are supplied by the customer when the conditions for zero rating are met, the main conditions are that the customer is a charitable entity using donated funds to purchase the medical equipment https://www.gov.uk/guidance/charity-funded-equipment-for-medical-and-veterinary-uses-notice-7016#zero-rating-goods-and-services-bought-with-charitable-or-donated-funds

Tim Xu ,   If you are selling goods in the UK then these supplies would normally carry VAT . However there are certain reliefs if you sell medical equiment to certain bodies. Please see the guidance below: Charity funded equipment for medical and veterinary uses (VAT Notice 701/6)     8. Evidence of eligibility for relief    . Thank you.