You need to keep the phrase of "I can pass my exams" firmly in your head. This can also be helped by keeping positive people around you, for example, if you feel that your parents are pressuring you to get very good results, then you should talk to them and tell them if you feel too pressured, and ask them to be more positive about you giving it your best shot, rather than the specific results you attain.
As stated above, revision seems to become a teacher's favourite word when the exams are looming and it is very important, but tips on 'how to revise' may not work when you revise in a different way to others, however no matter what your revision technique is, feeling prepared for your exams is important to thinking positively.
This can be helped by going through revision with teachers and seeing how much work you've done to prepare for the exams, thus giving you a physical sign that you should be confident as you've put the work in. As long as you put in the hard work, the rewards should come, just don't let panic throw you off.
Despite all of the revision you do, you won't know what the questions on the paper are until you're sat down and taking the exam and this can be a scary and daunting thought. However being more aware of this can help you, you can't really allow yourself to loose time by panicking that you don't know the question or the topic.
To be comfortable with the wording of questions, asking for past paper questions from teachers is the best idea as this will allow you to practice answering them, and seeing what work you need to do in order to answer them to your full potential. This way you don't have to fear the unknown.
You're Best Shot
The main way to think positively before, during, and after an exam is to be able to believe that you have given it your best shot, and nobody can ask you for more than your best. If you truly can believe that your effort during an exam was your best considering the questions and your amount of revision, then you can't get a better result than the one you will get.
I hope that this advice was helpful and good luck to anyone sitting their exams! :)