On the GRE you get credit for correct answers, but unlike other standardized tests, the GRE doesn't penalize you for incorrect answers. That means you should answer every question, even if your response is a guess. Of course if you know how to answer the question, answer it and don't guess. Some you will get; others you will miss. When you don't know and you have to guess, eliminate as many wrong answers as you can. Even if you cannot eliminate any choices as wrong, pick a letter, any letter. And even if you run out of time, fill in the rest of the answer spaces. You have nothing to lose by doing this and everything to gain.
The GRE consists of seven separately timed sections. The proctors will tell you when you can begin working on a section and when your time is up. Once a section is over, you can't go back to it. It's best to keep track of time yourself with your own timepiece. Even though proctors will inform you of time remaining, don't count on that. A proctor may or may not post the time at the exact moment it changes---for example they may say you have ten minutes, when you only have nine. Also, the proctor may post the time remaining when you aren't paying attention, so you may be working away and look up to see " 3 minutes". The solution is to bring your own watch. If you have one with a stop-watch, that would be the best. Just make sure to turn off the alarm. You don't want it going off during the exam.
Memorize the directions to each section before you come to the test. No extra time is given for reading the instructions for a section. If you have to read the directions, you are using time that could be better spent answering questions. The directions are printed in most GRE test preparation materials available from ETS directly. It will be to your advantage to know the directions to each section to save precious time.
Each section of the GRE is designed so that the questions get progressively more difficult. So how much time should you spend on any question? The analytical section for example contains 25 questions. To cover all 25 questions in the 30 minutes allocated you'd have 72 seconds for every question. Unfortunately, things don't work out so neatly. Some questions take less time; others take more. But knowing that questions get progressively more difficult will help you pace yourself.