Your first chance to play with cards from Foundations is at your local Prerelease. When you register for a Prerelease event, you'll get a special Foundations Prerelease pack on the day-of.

Foundations Prerelease Pack

6 Foundations Play Boosters
1 Traditional foil year-stamped rare or mythic rare
1Magic: The Gathering Arena code card; only available in select regions
1 Deck box
1 Spindown die (from among five total color versions)