In order to improve your written English, you have to read more books and write more. In order to improve your oral English, you have to practice your spoken English more with some English speaking people.
The following two websites are very useful for learning English and chatting with other foreigners on line. It is free. You may browse it when you have time. I hope it helps you.
http://www.englishbaby.com/lessons
http://www.eslmonkeys.com/student/esl_learning.php
Good luck in your School Cert. Exam.!