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Hello, you will first and foremost want to peruse this beginners guide http://moz.com/beginners-guide-to-seo. I do not rely on their on page optimization grader. Regarding the Grade "F" --  it says F because your page is not aligned with the keyword that moz software is grading your page for. ie your page ranks #1 for "farm raised pearls" but the page gets an F because the keyword moz software is grading you on is "farm raised pearl necklace".