There are several sites in here that can help you interpret your scores, once you get them:
If it's primarily one area that gave you fits (the editing portion, it sounds like), then you might go through some of the practice ACT tests and see how you do. Post the questions and potential answers and tell us which one or two answers you think are correct.
Here are a couple of the best sites I've found to help you study things about English sentences:
In Word & Sentence level, you'll find all the basic grammar including punctuation. In Paragraph Level, you'll find places to help make okay sentences better. Click on Index to look for other types of grammar/usage issues you are faced with.
Use the contents list at the left to navigate this site.