Regarding mobile phones, there are a lot of providers and possibilities. Like in most countries, you basically have the choice between a monthly call plan and a prepaid SIM card. If you opt for a monthly call plan, you normally have to sign a contract for a minimum period of 24 months. Your bills will be paid by direct debit. In return, you have to pay less for your mobile phone.
If you already have a mobile phone, you can also buy a SIM-only package. This pack includes only a SIM card, which you just have to insert in your unlocked mobile phone to be able to use the services. SIM-only packages are offered both for prepaid and monthly call plans. If you buy a SIM prepaid card with a certain call credit, you only have to insert it in your unlocked mobile phone and you can start to use your phone straight away. If you have used up your credit, you can upload your new credit quickly and conveniently, e.g. online, directly from your bank account or by buying credit at one of the sales points (i.e. petrol stations, mobile shops, kiosks).
As there are many different tariffs with special contract conditions, it is advisable to compare the different providers and to read the fine print. Some providers offer special tariffs for students so be sure to ask! If you want to use your mobile phone for calls and messages to your home country, you should enquire for special country packages.
The GSM 900/1800 system is the network standard for Germany. Note that it is not compatible with North American or Japanese systems (with a few exceptions).
Please keep in mind that driving while using a mobile phone without a “handsfree kit” is against the law in Germany.