It would help to know exactly which test you are taking, but I would guess you are going to have some construction, some reading comp, and a writing portion.
I think first, you should probably know grammar vocab (noun, verb, adverb, clause, etc.) and basic grammar rules. They will probably have you do a bit of "correct this sentence" or "re-write this sentence to fit new criteria" sort of questions. I'm not sure, but you also might want to know basic correction marks.
This is an english assessment study guide that will probably have good examples, but you should look for the study guide for the actual test you are taking.
Good luck!