Make Money Online from blog ?

[kuhashmi]

Hi, I am plaining to build blog but little confuse and can't able to decide in which topic i need to build a long so ican earn some money. I am plaining to build blog on olympics which will be held in UK (June). But after after Olympics ends will any body come to my blog, That's the question keep coming in my mind if i build a blog on olympics ?
Then what will be the topic i need to choice which gives me good money for long time.

Looking for good advice on that.