I will give you some basic study tips, and hopefull they will help you feel more comfortable with the information.
1 - List the key terms for the unit, with their definitions. Copy that list over a few times. Recopying notes is always a good idea - repetition is great memory technique, and writing them instead of just repeating them also reinforces the memory.
2 - Next, list the information for the unit in sections. Give each section a heading, and put the basic information under the heading. Do not make the sections large - you may need to reorganize the information into smaller groups yourself and come up with headings. By doing this, you will be manipulating the information and will better understand it.
Example - if the topic is cell division, don't have one section for cell division will all the information listed. Make separate sections for first step, second step, etc..
3 - After you have reorganized all the information, read over the notes.
4 - Have someone ask you questions about the notes. Testing yourself can build your confidence that you remember the information.
5 - Remember this.. if you have been in class and you have prepared for the test, then you have the information stored in your head. Stay calm and you'll be able to recall the information. If you come to a question you don't know, don't panic. You'll at least know something about the question. Make educated guesses... you'll be surprised how well you will do! Trust yourself.