Do keyword research. Google Adwords is a free and much quicker to use. You can also try Google insights. it also good since it provides a visual representation of regional interest on a map.
I usually use 2 more competitive keywords on my homepage while use some less competitive keywords for catalogs and pages. Use long tail keywords in my page contents everywhere.
By "competitive" you all mean Advertiser Competition value? If yes, then isn't it represents how many advertisers are bidding on it, not what the competition among pages for this keyword is?
Selecting right keyword is really so important so we should go for most relevat keyword that suits to your page higher chance to get clicked with low competition I mean to say it should be less compeitive with economic CPC.
Google Adwords is a free and much earlier to use. You can also try Google insights. It is also good since it provides a visual representation of area interest on a map.
Do keyword research.