If you are not VAT registered and your turnover is under the current VAT threshold of £85,000 in any rolling 12 month period, you will not currently have to register under MTD for VAT. It is optional to register for MTD for those who are voluntarily VAT registered. Once your turnover goes over this threshold, you are required to register for MTD for VAT and start filing under MTD.
You are responsible for monitoring your turnover and registering for MTD.
Note that adjustments like reverse charges increase your turnover limit and may push you into MTD.
Once you are registered under MTD, you are not able to leave the scheme.