You are able to withdraw cash (Euros) by using your credit card. There are many banks and cash machines.
Notice that it is not always possible to pay directly with your credit card. Electronic payment is not as usual in Germany as in the U.S., so it may be useful to take cash with you.
In Germany you give 5-10% tip, depending on how good the service was. If the waiter brings the bill Germans typically either give him the exact amount of money (including tip) they want to pay and say “Das stimmt so.” or (if they don’t have suitable notes and coins) they tell the waiter the amount they want to pay (including tip) and get change. If that sounds too complicated you can also just leave the tip on the table.