The U.S Studies Library at Johannes Gutenberg University offers a wide range of materials to help you prepare for English tests (including the TOEFL Test). For more information visit the U.S. Studies Library´s website.
In addition, Johannes Gutenberg University offers language courses for students. Further information can be found on the website for the center of foreign languages at Johannes Gutenberg University.
Besides offers from the university itself, there are many commercial providers who sell preparation materials and practice tests.