You can do this using the Kindle app. More information may be found here:
How to Install the Kindle Reading App
Basically you'll need to download the Kindle app on to your Galaxy Tab. You can find the Kindle app in the Google Play app store.
You'll need to make sure the Kindle app is registered. When you open the app for the first time, it should ask you about registering it with Amazon.
In the future when you download Kindle books, you will have an option to send them to the Kindle app on your Galaxy tab.
