Do I need a card to start the trial?

Yes. The 14-day trial starts once a plan and a payment method are chosen, and nothing is charged until the trial ends. The exact date of the first charge is shown at signup and again in Settings, Billing, so there is no surprise about when it happens.

The card is asked for up front so that the day the trial ends is the only thing that changes: your store stays connected, your costs stay priced, and nothing has to be set up twice. Cancelling during the trial ends it with no charge, from Settings, Billing.