You'd need to log in to your DigitalOcean account, click on your avatar, and then:
Settings => Billing
From there, you can add a credit/debit card, or top off your account using PayPal. If you already have a credit/debit card on file, and it's valid, DigitalOcean will auto-charge that card at the first of the month.
If you don't have a credit/debit card, you'll need to top of your account using PayPal before the first of the month to ensure that your account has enough credit to cover your usage.
How if during set up is not set up by me and i dont know the digital ocean account user name and password?
If you're not the account owner and/or the account owner hasn't granted you access to the account or a team, you wouldn't be able to login and make a payment. The account owner would need to do this on their end, or they would need to grant you access to a team with your Droplets in it.