A phone and Internet contract for the mobile phone can be concluded with different providers. There you pay a monthly contribution, for which one gets a free contingent for calls, SMS and Internet. You can compare the providers for example www.verivox.de or www.check24.de.
In addition, there are prepaid cards with which you can make calls for the charged value and use the Internet. You can buy the cards directly at the checkout in various supermarkets, such as Aldi, Lidl or Netto.