I am confused every time when I am using these words, then using any one of them without knowing its difference. Can anyone help me how could I use these words in appropriate situation?
The alleged American (-ize) vs British (-ise) usage tells an incomplete story, more on which can be found here: http://www.metadyne.co.uk/ize.html
Suffice to say that the z-form has a long history in English.
To summarise (ha!) my own experience: Whilst at the University of Cambridge in the mid 90s I would consistently rely on the z-form unless the word came directly from French, probably because my dictionary of choice through school was one printed by the Cambridge University Press, I don't know.
After a return to the UK after twenty years in an academic environment in the United States, I have reverted to the s-form, deliberately over-emphasising my Britishness.
Above all else, be consistent.