In case you have been selling goods before being VAT registered in a country, a backdated registration has to be initiated. Depending on the country, there are different conditions and rules. Please read more info here.
If a backdated registration can be processed, you will have to file backdated returns with the Tax Authorities. Please see a list of the countries with their requirements below.
Be aware : for late filings and/or payment, penalties can occur. Read more about this here.