The new regime begins on 1 April 2019, however the exact date from which you must be MTD for VAT compliant depends on what your VAT periods are:
If your next VAT quarter end date is 31 March 2019, or you file monthly, you must comply with MTD from 1 April 2019.
If your next VAT quarter end date is 30 April 2019 you must comply with MTD from 1 May 2019.
If your next VAT quarter end date is 31 May 2019 you must comply with MTD from 1 June 2019.
If you become VAT registered after 1 April 2019 due to reaching the turnover limit, you must comply with MTD from your registration date.
However there is a small group of businesses who do not have to join the MTD for VAT programme until 1 October 2019 at the earliest and they are listed on GOV.UK. The list includes businesses who complete annual VAT returns. HMRC have written to all businesses who must join MTD for VAT on or after 1 October 2019. If you think you are within one of the groups but have not had a letter confirming this from HMRC you should contact the VAT helpline.