You're always required to provide a credit card or a bank account in order to verify that you're not a bot (otherwise imagine the spam that they would receive).
Refer the FAQ (https://cloud.google.com/free/docs/frequently-asked-questions)
You won't be charged until you upgrade to a paid account. Google has limits on all of their free tiers to prevent users from going over thresholds.
Note that the Free Tier and Always Free offers have different requirements:
The first is a trial (with a time limit and credit limit) that will ask you to upgrade to a paid account once it's over.
The latter allows you to use cloud services without paying up to a certain usage limit, after which you will be billed at their normal rates. You're only eligible for Always Free if you already have a paid account.