The following websites contain valuable guidance to improve your writing. You might want to check them out before taking the English Placement Exam.
The Purdue Online Writing Lab (OWL)
"The Purdue University Online Writing Lab serves writers from around the world and the Purdue University Writing Lab helps writers on Purdue's campus."
Guide to Grammar and Writing
Basic Guide to Essay Writing
"A step-by-step guide to writing a basic essay, along with links to other essay-writing resources."
Grammar Girl Quick and Dirty Tricks for Better Writing
Just for fun! "Grammar Girl provides short, friendly tips to improve your writing."