I have NO IDEA WHAT I’M DOING
Don't worry that's very common.
I have 2 degrees, that's not to brag its just to let you know that's 8 years of studying for exams + an extra 3 years from highschool. Studying can be daunting as you try to memorise every thing on the subject. The important thing is to remember you are studying to pass an exam.
You should not be trying to memorise the entirety of biology. Just the things which will help you get a good mark in your exam.
You need a strategy for passing exams. 
1. Understand the type of questions that will be asked.
2. Workout answers for these questions in advance.
First thing is to get hold of past exam papers, they should be available through your school of just google them.
When you look at the exam papers you will see
the same type of questions come up again and again. This will give you good practice at understanding what the question is asking and the type of answer they are expecting.
Practice writing out these answers again and again. 
It's easy to lose a lot of time in exams trying to put your thoughts down on paper clearly that's why its important to practice you answer so you will know what your trying to say before you even say it.
When you sit the exam
the questions wont be exactly as you have practiced but close enough that you should feel confident in being able to answer then by adapting the answerer you have already prepared.
EXTRA MARKS IN EXAMS!
Getting the answer right is fine but you usually get extra marks by knowing something extra that was not taught in the class room.
A good example of this if you have been taught photosynthesis and its a question on the past papers, read any article on it and quote that in you answer showing you have additional knowledge and understanding.
Exams are tough they are stressful and you really don't get taught any coping strategies for them.

Like most things in life you have to discover for yourself what works best for you. I have found these tips help in taking away the anxiety and stress involved in studying for an exam.
Hope they help.
