Google Adwords Keyword Tool is excellent to get you started on Keywords. However, that really is only the start. After you identify which keywords or phrases get a reasonable amount of searches you then need to research the competition for that keyword. thats the part that takes the time. It can be dome on a manual basis but there are some excellent paid tools out there as well. It all comes down to basic math. Suppose you used a keyword which had 50,000 searches per month, but the competition for it was high (lots of indexed pages, lots of authority sites in the top 10 results, and pages in the top 10 results with PR above3), then you would find it difficult to ever make the first page, let alone the top 5 results. On the other hand a search term with 5000 searches might have must less competition, which makes it easier for you to rank in the SER.