Go System->Configuration->Google API->Google Checkout
Enable Google Checkout and other configurations set details about your google checkout like merchant id, merchant key etc. Save the configuration.

You Can Go to google checkout sandbox testing platform and can easily test about google checkout

you need to have a seller merchant account and buy account to test it both of this account can be created from   google checkout sandbox just provide sample or proxy data of the users and after this you are ready to test your google checkout integrated with magento  on the sandbox platform with no real money transfer but it will stimulate the exact scenario after test you can make it live but changing the configuration to live from backend.