A good conversion rate from ad words is not just choosing relevant key words; the most important factor will be your website. An agency will have valuable experience in this field.
By the time you have read this site, you will already have a much better understanding of Ad words and its mechanics, including negative key words, articles and how important they are, and different keyword formats. There is still so much I have not mentioned, including text ads, click through rates, importance of landing pages and much more.
My advice is, that if you try this your self, it will be a costly experience, your time is better spend doing what you do well, running and developing your own business. Using an agency will definitely be far more cost effective for you. A profitable ad words campaign takes experience and time.
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