Go to your dashboard, click billing in the bottom of the left menu. Then click add card under payment methods at the bottom. Payments are taken automatically I think, at least mine are.

Hi Leo10m2010 - Not to worry, our billing does not happen immediately upon usage.

We invoice and charge your credit card on the 1st day of each month for the previous month’s usage. Time zone differences can cause some minor variation in invoice date and time. DigitalOcean bills in USD. To keep our pricing stable and consistent, rather than fluctuating with exchange rates, we do not bill in local currency. Similarly, we do not invoice in local currency. All invoices are in USD.

You can read more about our billing practices in our Product Documentation - https://www.digitalocean.com/docs/accounts/billing/