in which federal state,are you living? This is important to know, because it may differ from state to state. I give you an example for Bavaria:
After Realschule you can attend the Fachoberschule to achieve the Fachabitur (access to a university of applied sciences/polytechnics).
Realschule students who leave school with a good graduation certificate can take the opportunity to acquire the general university entrance qualification at a Gymnasium (access to university).
But there are even more possibilities. If you'd like, and tell me in which state you are currently living, I can research for you.