Newbie Question - from an old timer

Hi all,
I'm about to ask what might seem like a really green question but let me just preface it with the phrase - I've been away for a few years now and previously spent all my time at the warrior forum (and as we all know that place is not actually great for advice - especially if you're talking to someone who wants to sell you something).

So - can someone tell me a good way of doing niche selection (preferably evergreen)?

Thanks in advance for humoring such a newbie question.
BPS.