Check out Jeandevaches' Guide to the ACT (Just search the forum for it). It has some simple and helpful tips to help you increase your English score. Check out SparkNotes because I found the grammar review to be helpful. English is a section that has come naturally to me, so there is not much specific advice I can give. If you are really struggling with English, memorize grammar rules and then take ACT practice tests out of The Real ACT Guide (The Red Book). Also search the forum for "ACT Practice Tests" and there will be links to official ACT tests in the different threads.
Good luck on improving your English score!