When you are writing your hub, look for the hub capsules at the top right of your editing page. Click on the Amazon one and it will add an Amazon capsule to the bottom of your page. Type in your keyword (just one, more won't work) and tell it how many products and what type (books, electronics, or etc.) and it will make you a display of your Amazon products with descriptions and prices. If you don't like the looks of the products, experiment with different keywords or product categories, or numbers of products displayed, until you get one you like. To change the position of the ad capsule, reorder them at the right side of the page. To make them smaller, double-click on the capsule marker on the right column of the page and it will make it smaller - add an Amazon (or any other) capsule just above a text capsule to make the 2 capsules come out side by side.
First, however, you must go to amazon.com and register and get accepted as an affiliate. Then go to your affiliate linking page under "my account" and enter your "ID number" from Amazon so you will be correctly credited at Amazon for your sales, as Amazon is who will pay you for those sales.
Good luck! :)