Billing setup is part of the account creation process. If you have set up your billing options, you should be able to check on your status in the BILLING tab of your account.
More information on billing is available here: http://support.google.com/adwords/bin/answer.py?hl=en-GB&answer=1704417
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All regular payment methods such as credit cards are accepted, click on the Billings tab on the upper right and fill in the blanks.
The Amount of money is calculated with your bids.
If you don't have enough money to pay the bills, try "Conversion Optimizer"