In preparation for KET exam:
Go to http://www.cambridgeenglish.org/exams/key-for-schools/preparation/ and practise in preparation for the exam. Note that the answers/key are also there-so have them close by and check your progress as your practise.
Scroll down to vocabulary list. Work through pages 6-8, noting down the words that are new to you. Write them out and translate them. Use dictionary.cambridge to look for how they may be used in sentences. Make your own sentences with them, making sure you know how to use them in a real life-context.
Ana and in preparation for PET exam:
Do the same as above only with the PET level http://www.cambridgeenglish.org/exams/preliminary-for-schools/ .
Cover first 4 pages of the vocab list (pages 5-8)