Entering the country from abroad
As a matter of principle, citizens of a third country who would like to work in Germany and are currently still abroad need a visa for work purposes in order to enter the country. This also applies as a rule to cases in which it would be possible to enter the country for a short stay without a visa (see below for the exceptions). It is important for you to enter with a visa which corresponds to the actual purpose of your stay, as this is the only way to ensure that your visa can be easily extended or changed to a residence title by the competent aliens authority (Ausländerbehörde) in Germany.
A visa which has only been issued for a short stay, such as a tourist visa, can as a matter of principle not be used for a long-term stay. In the same way, it does not entitle you to work. You need to apply for your visa in your home country before entering Germany. Please make the application with the German representation abroad that is responsible for the place where you live (Embassy or Consulate-General). Please ascertain in good time there what documents you need for the application.
Exceptions from the visa obligation apply for a stay which is not only short for nationals of Australia, Israel, Japan, Canada, the Republic of Korea, New Zealand and the United States of America (cf. section 41 subs. 1 of the Residence Regulations). If you may enter without a visa and are planning an extended stay in Germany, you must apply for a residence permit to be issued within three months of entering the country (section 41 subs.3 sentence 1 of the Residence Regulations). This application is submitted to the competent aliens authority (Ausländerbehörde). Which aliens authority (Ausländerbehörde) is competent depends on where you will be living in Germany.
Further information: see brochure entitled "Studying and Working in Germany", pages 6 et seqq.