Recommendations:
The test includes a wide range of media knowledge from the difference between a warp and weft (weaving) to lenses for photography and glazes for ceramics. There are only one or two questions on each media type. You should be familiar with the stages of artistic development, the elements of art and principles of design and some very basic art history. The short answer portion asks you to look at and analyze works of art. It's okay if you can't ID the pieces exactly. The prompts are worded in such a way that you could get by making a formal analysis of the piece without knowing everything about it.
Resources:
Visual Arts MTEL Practice Test
Book Recommendation: MTEL Visual Arts Exam Secrets Study Guide: MTEL Test Review