There are two ways to look at this question.
First, if you do not understand what the assignment requires from you, you should ask your instructor. Instructors have the responsibility of making their assignments clear to their students so that students can understand what is expected. If you do not understand, contact your instructor during office hours or by email.
Second, if you understand the assignment but don't understand what a good answer would entail, you have at least two options. First, you can ask your instructor for help. You should explain what you understand and what you don't understand so the instructor can help you. Second, you might consult someone like a reference librarian. You can explain what you are trying to accomplish to the librarian who can help you to find sources that might help you complete the assignment.
The best way to get more information is to talk to your teacher. If this isn't possible, ask some of your classmates. They might have heard something you didn't. After that, look on the internet. My physics teacher refuses to explain the labs to us until after we turn in the lab notebook. Looking up the information online and discussing it with classmates helped salvage my grade!
I would say the best thing is to ask the teacher because they could help you the most. If the teacher does not help you but you find students that are completing the assignment, you could ask them for help too. The best thing to do is ask the teacher though, they will give the best answers.
Like the previous answer, I think there are two approaches to this question but mine differ slightly.
Firstly, if the question is a basic question not regarding the content of the assignment, the student can ask his or her classmates for clarification. This includes questions like the due date and formatting. The teacher will have most likely gone over this information before (probably while assigning the work) and going to the professor with these basic questions will not always be warmly welcomed. But of course, to be safe, it would probably be best to ask around with several students to make sure of the answer.
On the other hand, if the question is deeper and more closely related to the content, then I would suggest asking the teacher. Examples of these questions might be guidance as to the direction of the assignment or the level of depth expected from the student's work. Otherwise, if the student is very worried about the assignment, maybe do a check up with the teacher. For example, if the instructions for an essay were very vague, it could be helpful to ask the teacher to glance at your thesis to see if you are heading in an appropriate direction (if the teacher will allow it of course!).