If you have not yet been admitted to study at a university in Lower Saxony or to Niedersächsisches Studienkolleg, you should apply for an Applicant Visa ("Visum zum Zweck der Studienbewerbung" §16 (1a) German Residence Act).
This allows you to enter Germany for three months, in order to meet the admission requirements for a study programme in Germany (e. g. taking a German course). You may extend your visa for up to another six months, if the original period turns out to be too short to meet the requirements. Overall, the maximum length of the stay for application purposes is nine months.
If you are admitted to a university or to Niedersächsisches Studienkolleg within this period, you can apply to your local Foreigners' Registration Office to have your visa converted into a residence permit for study purposes.