To help your student prepare for an exam, you must prepare as well. Know in advance when the exam is, what the exam is covering, and where your student is currently with the material. Alter the structure of your lessons accordingly. Often it is helpful to give your student a practice test on the material they are being tested on. You do not need to design your own practice exam, there is an abundance of resources online. Simply Google what you’re looking for, and you will likely find it.
For example, if a student of mine is in Algebra 2 and is being tested on factoring methods, I can Google “Algebra 2 factoring worksheet”, go to images, and pull a worksheet quite easily. Or alternatively, you can design a simple worksheet at websites like easytestmaker.com