If you'd like to know which of your keywords best leads to sales, you should integrate your Google AdWords account with your store .

How to set up AdWords Conversion Tracking

1. Go to your Google AdWords back-end and open the Tools and Analysis → Conversions page. Press the New Conversion button. Enter your conversion name and press Save and continue.

2. Set up conversion settings.

• Set "Conversion category" to "Purchase/Sale".
• Set "Page security level" to "HTTP" or "HTTPS", if your store page uses a secure HTTPS protocol (most likely it does not)
• Set "Conversion value" to %order_total%, it will allow you to dynamically track each sale value.
• Set "Tracking indicator" to "Don't add a notification" if you site's privacy policy mentions that customers' actions are tracked.
• Press "Save and continue".

3. Select the "I make changes to the code" option and copy the code snippet to your clipboard, or save it to a text file. Press Done.

4. Log in to you control panel and make sure that you are on a paid plan which allows you to embed tracking codes to the last checkout page. Open the System Settings → General → Cart page and find the "Checkout options" section. Enable the Affiliate code on "Thank you for your order" page" option.

Then enter your saved tracking code and save the changes.

A detailed instruction is also available at support.google.com/adwords/bin/answer.py?hl=en&answer=1722054

Having trouble seeing your conversion code? click here