First of all you should know, that there are different types of visas in Germany. Most important for international students are the “student visa” (in German: “Studienvisum”) and the “language course visa” (in German: “Sprachkursvisum zum Studienzwecke”).
A student visa you can only get, if you have an admission for a German university. If you do not have an university admission yet, because you want to learn German and prepare the German language exams before you start your studies, you can receive a language course visa.
For the language course visa German missions abroad usually want to see proof that you are a student and a confirmation, that you are inscribed in a language course in Germany, that is preparing you for university studies. That means your language course has to be an intensive course with at least 20 lessons a week.
Apart from that you’ll also need a confirmation (in German: “Finanzierungsnachweis”) that you have enough money to pay your bills in Germany. Please contact the local German embassy in your country for detailed information. Here you can find an alphabetically ordered list of German missions all over the world.
To get your language course confirmation you’ll have to register for a language course and pay the course in advance. After registration and payment you’ll receive your confirmation from the language school. With this confirmation you can go to the German Embassy to get your visa.
With a language course visa you have enough time to learn German for university studies even if you start from zero. Remember that a language course visa is only valid as long as you attend German classes. If you cancel the course the visa will expire.
If you are looking for a language school in Germany, please check out my article “Where can you learn German in Germany?” and contact a language school in Germany directly.