If you use Wordpress as content manager, just install the boo-box plug-in and follow the instructions over there. Blogs hosted at free service Wordpress.com can't have boo-box (yet).
If you have a boo-box installed, uninstall it.
To install the plug-in get the zip file and unzip it at folder /wp-content/plugins. Then go to Plug-ins page at your WordPress and activate it. Notice that a boo-box tab will be available in the Options menu, where you will be able to choose your store and place your associate ID.
boo-box for b2evolution
Who use b2evolution as blog manager can get the boo-box plug-in for b2evolution, just follow the instructions at plug-in source code.
If you have a boo-box installed, uninstall it.
To use the plug-in, get the zip file and install it at your blog.
All blog managers
As promised, boo-box’s installation is very simple.
Copy the one HTML line below and put it between <head> and </head> tags in your blog’s template.
With text is almost the same thing. You'll use span in the text you want to be linked, keeping the boo-box-link term in its class. Look:
<span class="boo-box-linkoakley sunglasses">my super cool Oakley sunglasses</span>
If you use specificy tags in your image and text, your selection of products will be better and more related to your content, and then your audience will have more chances to buy it. Get some tagging tips here.