Start with the free official Goethe Zertifikat B1 Modelltest PDF from Goethe.de. Then buy the Hueber book for extensive practice.
Use the PDF's answer key to see sample essays. Pay attention to connectors (zunächst, außerdem, zusammenfassend).
If you only download one PDF, make it the official Goethe Modellsatz 1. It reflects the current exam format perfectly.