To study in Lower Saxony you need a so-called "Higher Education Entrance Qualification" – in German: "Hochschulzugangsberechtigung" or "HZB" for short – as proof that you are qualified to take up university studies in Germany. If you are a citizen of an EU-country, this can be confirmed by your school-leaving certificate, provided that it qualifies you to study in your home country as well. This case is known as "direct university admission."
For a non-binding assessment whether your school-leaving certificate will be recognised as a higher education entrance qualification before the actual application, you can use the Anabin online admission database (in German only). Simply enter the country in which you have received your school-leaving certificate.