Once you received your personal SIM card of the provider of your choice, you can buy credit for your phone number. You don’t need credit if you just need to be called by other people, you can even use the SIM card with 0.00€ credit. However, if you want to activate a certain plan, you will have to top up your SIM card first.
You can either buy credit in any German supermarket, at gas stations or at the mobile phone provider stores, or you can buy the credit online.
Buying the credit online is particularly convenient if you need to start using your phone right after your arrival.
Here are the links for the providers’ online credit store:
Congstar: Buy credit online
Telekom: Buy credit online