Should I guess on the SAT?
Guessing on the SAT did not used to be particularly advisable, unless you could eliminate at least one of the answer choices as wrong. The test incorporated a penalty for wrong answers such that anyone guessing at random could expect to lose four fourths of a point from wrong answers for every one point they gained from a correct answer. The "guessing penalty" has been eliminated, but the general approach to guessing should remain the same: for any question for which you don't know the answer, you can improve your chances of answering correctly, and thus gaining a point, if you eliminate as many wrong answers as possible before guessing randomly.
Is the GRE like standardized tests I've had to take before (like the SAT or ACT)?
There are of course, some significant differences between the GRE and the SAT and ACT, but the good news is that the overall experience of taking the GRE is not worlds away from that of taking "earlier" exams. One basic way of characterizing the GRE in comparison to the SAT, for instance, is that the GRE heavily tests vocabulary in a difficult way, which the SAT has abandoned, but the SAT tests math concepts and skills of greater complexity and difficulty than the GRE.
What's the difference between the Reading, Writing, and English sections?
The skills and abilities assessed by these portions of the ACT are closely related to one another. In fact, an English Language Arts score is reported as a sort of sub-composite of these sections when a student opts to sit the essay for the Writing section. In the Reading section, you are tested on your reading comprehension, asked to make inferences and draw conclusions about the meaning, intent, and implications of various passages. Whereas the Reading section asks you to read analytically, the Writing section asks you to write analytically and persuasively. In this optional section, you are tasked with responding to a prompt by taking a stance. You are graded on the skill with which you clearly and convincingly develop your perspective and contrast it with a competing one. Finally, the English section asks you to demonstrate your grasp of the elements of writing style. The questions pertain to wording and grammar choices and how they may be effectively used to increase clarity of meaning and message.