deciding the topic/niche?

[sunshine]

Hi,
Well, i want to ask you, expert IMs, how you choose the niche, i mean, i want to make a site for adsense income but not sure how to choose the topic, yeah i know, i should choose the topic what i love and have passion for, but the things/niches i am much passionate are highly competitive, like self help, ebooks, starting own business offline online, self development, marketing, luxury and so on.
So let me know how you decide, when you are in same situation, i know many big IMs have tapped the niches they have niether knowledge about nor passion for, just income factor is there!

Well, i know there are some tools to help me, like overture's tool, google external, nichebot,... and some paid services like #*$!, but i am here asking you for a solution which is free like the tools i wrote, but i am afraid how to use them effectively,

like overture tells the demand, but how to analyze the supply side (again free please), how to know the high paying keywords to traget,....... i am much sure you the big guys will reply with a great suggestions!

Thanks in advance!