It's the same Amazon you know, with the same products and prices, but with the chance to give while shopping.
Just use your usual Amazon account (username and password). This sets up your AmazonSmile account.
You will be asked to select your charitable organization OR search for one. Type "Cartwheel Foundation International" in the search bar
You will be taken to the main Amazon Smile marketplace. Make sure it says AmazonSmile in the left hand corner of the search bar, and that 'Supporting: Cartwheel Foundation International' shows up below the search bar.
Tell your friends about Cartwheel and AmazonSmile. Just mouse over your supported charity and a hover box will appear that lets you share your support via Facebook and Twitter.