Calling a mobile phone in Germany is just as easy as calling a landline. The only difficulty you could face is that there are no official directories for mobile numbers in Germany, so you need to know the number of the person you want to call. Some people publish their mobile number alongside their landline in the telephone directory, but this is the exception rather than the rule.
Dial the international prefix from your country, followed by the country code for Germany -- which is +49 -- the network code and the person you are calling's personal number. Germany has several competing mobile networks, but every mobile dialing code there starts with 01. Drop the 0 when you call from abroad.
Dial the international prefix 011 if you call from the United States. From most other countries, it is 00. For example, if you want to call a German mobile from the U.S., the number would look something like this: 01149-1770-123456. From most other countries it would be 0049-1770-123456.
Be aware that a mobile phone keeps its number and country code, no matter where in the world it is. For example, if you want to call an English mobile phone whose owner is currently in Germany (or anywhere else in the world), you need to dial the country code for England (which is +44), then the number.