The ACT test is a standardized test that students can take in the US. This test shows whatever prowess a student may have in taking standardized tests, and colleges will almost always look to these scores to help decide whether or not a student will be allowed to go there.
The ACT is composed of four sections: Writing, English (Grammar and mechanics), Math, Reading, and Science. Each section is graded out of 36 points, except for Writing which is graded out of 12 (up to 6 points each from 2 readers). Personally, the hardest sections I've found are the English and Writing sections. Math is limited to basic algebra and science is based on a small amount of background knowledge coupled with logic. Reading is a little bit trickier, but any competent who has taken a literature and composition course at their high school should know the basics of understanding literature and how to find key details in a story.
The English section is entirely about grammar. From my experiences taking the test as well as practice tests from workbooks, around half of the questions can be answered by choosing the least wordy answer and/or choosing the one that sounds best. While this may be fine for some, one must know the actual rules in English to achieve a score above a 30. The only advice I have for my peers taking this section is to simply do a lot of practice tests and learn from your mistakes each time. The first step to getting better is figuring out what you're bad at.
Writing is the first section on the ACT. While technically it is optional, I would highly recommend against not doing unless you already have a 12. The most important thing I have learned for this part is that length matters. Filling out all the pages that is provided will guarantee a student at least a 9, so long as everything still makes coherent sense. For those less able to write so much in a short amount of time, a solid thesis statement will net you a 4. When it comes to filling out, I try and get at least 3 lengthy body paragraphs, or 4 somewhat shorter ones. Coupled with a decent introduction and a conclusion, length should not be a problem so long as you have something to write about.
Here are some tips from the ACT site.