Battlepasses are unlocked with Raider Tokens, the premium currency.
Items, skins, and Raider Tokens within the battlepass are unlocked with Cred.
Cred is earned by completing battlepass challenges. You'll receive 3 challenges at a time each giving 3 cred. You can see your progress in raid with the Challenges tab.
When you complete the first challenge in a 24 hour period you'll receive an extra 30 cred, then 20 more cred at 3 challenges, and 10 more cred at 5 challenges.
*Tech Test 2 had 3 battelpasses simultaneously but we don't know how many will be available at launch/during betas