It's perfectly described here how to do it, the only problem: He doesnt know the function openFileOutput();
private void saveSettingsFile() {
          String FILENAME = "settings";
          String string = "hello world!";
          FileOutputStream fos = openFileOutput(FILENAME, Context.MODE_PRIVATE); //openFileOutput underlined red
          try {
            fos.write(string.getBytes());
            fos.close();
          } catch (IOException e) {
            Log.e("Controller", e.getMessage() + e.getLocalizedMessage() + e.getCause());
          }
}
Those are the relevant packages I imported:
import java.io.FileOutputStream;
import java.io.IOException;
import android.content.Context;