You mean a pastel? You need to bleach your hair to as pale as it will go, pretty much white blonde. That is very hard to do and requires multiple dye jobs if you don’t already have relatively pale (and I do mean, pale) blonde, and also toning your bleached hair before you dye.
After and if you get it light enough you can use a dye, like Special Effects or Pravana (those are the only ones I recommend, and I have recommended them multiple times, we have a hair tag). You don’t necessarily have to have a pastel dye, as long as you dilute a ‘regular’ dye with white shampoo it will be light enough to apply as a pastel.