They say genius is 90% hard work and 10 % talent. It's the same for exams. If you didn't get any of them you need to ask yourself what the problem was. Did you understand it? If not you need to ask your teacher or your friends to explain it to you. ( Good revision for your friends!)
If you did understand it then the problem is you didn't spend enough time learning your work and / or your technique is poor.
You can't revise and watch television, listen to music, talk with friends, make coffee etc. You can't study late at night. You need total silence.Go to a library if your home is noisy.
If you find your mind wandering start at the beginning again. After a few tries at this you will have abetter idea of the amount of time you are going to need to revise. It will be lots more than you first thought. Next look at the chapters in your text book and work out how much time you need to allocate for each one. Plan your revision time. So, I need 1 hour to do this section of Maths ( Monday 5-6. 1 hour for this section of psychology Monday 6-7 etc.) A change is as good as a rest.
Start NOW. It is absolutely wrong to think that you'll remember it better if you leave it to nearer the time. Nearer the time is when you put the final touches to your revision NOT START!
If you didn't get your exams last time you are going to have to find more time than everyone else. Imagine you are in training for a race. You must eat well ( no alcohol) you must have plenty of sleep at least 9 -10 hours night. The best time to study is first thing in the morning so get up at 6am say and get an hour in before school.
Go through your work / text book , read it half a page at a time and then write down what you can remember of it. Write down questions on each page , re read the page and try and answer the questions. You'll need to do this several times before you know it. Be honest with yourself NO CHEATING.
Order past papers for your exam board and go though them ALL without looking at your notes. If you can't do them you haven't done the work. You should be studying say 3 hours in the morning, 3 hours in the afternoon, 3 hours in the evening. Mon-Thurs. Friday evening off. Study Sat and Sun morning at the very least.
Ah I hear you say I haven't got the time to do all that.............then you will fail AGAIN!