I want to learn the "keyword", where should I start ...?
keyword to get web traffic
There are a lot of free keyword tools and the ones above are great examples, and you can also check out SEOToolbook's keyword research tool: http://tools.seobook.com/These tools will give you examples of keyword demand, related keywords you can potentially target and how much traffic they can bring you. Take a look at this keyword research guide from SEO Moz, it will really help you: http://guides.seomoz.org/chapter-5-keyword-research
Based your "keywords" on your target. Some good tools online: Google insights for search, Google Instant & Wordtracker. Happy keyword research!