Things to Consider while doing Keyword Research!

Hi friends! I want to know few things about keyword research. Recently I've been doing a keyword research on green niche and while doing research I'm bit confused :o
So i just wanna know what are the things while doing keyword research ? I mean how much be the average search volume, competition, and cpc to calculate a relevant and effective keywords....All that things i don't know what to consider. So any of you guys who are expert in keyword research please give me your helpful suggestions on choosing very relevant and effective keywords which suits average green website.
Thanks!