The easiest way to use your mobile phone is by purchasing a prepaid card. This is the only option available if you have not got a residence permit yet. Choose a provider and visit one of their branches with your passport or ID. You can then buy a SIM card and charge it with a certain amount. After purchasing a prepaid SIM card, it only takes 15-30 minutes until you can use your phone. Some providers offer special tariffs for phone calls abroad. It’s worth comparing different offers, for instance by using a free comparison service.
If you'd prefer a contract with a telecommunications company instead of a prepaid solution, you first need to get your residence permit. Please bear in mind that even then some companies may ask for a deposit depending on your type of permit and your country of origin.