In summary
1) You must focus on your goals and have a plan in place. With a step by step process on how you are going to achieve it. Focus on what it takes to get to that goal and cut out the other things that are unneccesary.
2) Learn how to properly introduce quality studying into your routine. Do not focus on the amount of time studying but rather the quality of time studied. This means not on your phone while studying but full focus.
3) Learn how to properly study for each type of subject. Understand that it is impossible to try to know everything. You must focus on what is the tested material and the most important core material. Learn the study technqiues such as active recal and space reptition for subjects that require memorizaiton like biology. So that you are able to memorize it well. While for other subjects such as phyics and math try to understand the you must understand the material first. After establishing an understanding then practice and memorize the material. Try doing practice quesitons and exam quesitons and try all the different types of exam questions. Once grasping a firm understanding and praciticng all types of questions you should be able to do the exam.