You can use Keyword Discovery free keyword search tool and Google adwords keywords tool. They works great.
actually i dint understood what is that meters in green showed in that google adword tool when i search a keyword ??
Do you mean the biggest traffic driving keywords? While the external keyword tool is useful to find big traffic drivers it is important to remeber that these are often REALLY competetive. Perhaps you could concentate on more specific longtail searches to begin with - especially if your site is brand new.