Birthdays are important in Germany. Adults as well as children celebrate their birthday with family and friends. The person celebrating his or her birthday receives flowers, a cake with candles, and gifts.
If it's your friend's birthday, you should wish them a "Happy Birthday!". You buy a present only for good friends, or if you've been invited to a party. If you don't know what you should get the person, it's okay to ask them if they have a particular wish. Many Germans use the English expression "Happy Birthday!" rather than the German "Herzlichen Glückwunsch zum Geburtstag".