the rule is to find niche with high search volume and low competition , actually this a big mistake , any many new e marketers fall in this mistake you should find a great niche with high competition and low competition for keywords , for example , a niche like weight lose , weight lose is one of the highly competition niche , but you should find key words with low competition and high search volume ,for example " how to lose your weight in 7 dayes " , this will be the targeted keyword .
There are many places where you can find excellent long-tail keywords. You rely on Google keyword tool, or other free services… and there are plenty of them.