In restaurants, bars and even when you get pizza delivered, it's common in Germany to give a tip. About 10 percent of the check is normal, but you decide whether and how much to tip. If you were very unhappy with the service, then you do not have to tip at all.
If you give the exact amount you want to pay (the check total plus the tip), then you say "Stimmt so" ("We're good" or, literally, "It's just right"). Then the waiter knows not to give you any change. If you do not have the right amount in bills or coins, then you can give them the money and say what you want to pay (check total plus tip), and the waiter will give you the change for that amount. For example, the check is 18.20 euros, and you want to give 20 euros (check plus tip), but you only have a 50 euro bill. You say "20 euro, bitte" and the waiter will give you 30 euros back. If you do not say an amount, the waiter will give you back the exact change, without a tip.