Have you got any notes from your IGCSE?
Practise options from Ms Word and Power Point. Go through all the different features. Check out the options of paragraph, indentation, margins, borders, headings etc.
And for Basics of Computer, surf the net for notes on Input-Process-Output system, RAM and ROM, components of the computer etc.
What sort of questions were you asked during the class test? Maybe, you'll be tested the same way again. So, study accordingly.