See, Google and Bing (and other search engines) want to know what your images are. It helps them determine if your web page is the one that each searcher is looking for. If Google is scanning your site and seeing that there are images that have no alt text, they have nothing to go on. Also, a reason to have quality alt text is that if someone is viewing your website (this applies to emails as well) and has it so the images are turned off, they'll have no idea what the image is. Knowing what the image is makes it more likely that they will want to learn more about what you're offering.
So, how do you do it? It depends on your website or blog. It may be in a simple visual way or you may need to just put in the code. Many of the template sites or blogs have this option. Here is screen shot of Blogger's alt text option.
Is the image a new product at your shop? Say what it is, like: "New Coffee Travel Mugs from Cafe All the Way" or if your image is for a sale, be specific, with "25% off through March 15th on all Amy Butler fabrics".
Be clear and truthful with your tags and definitely make sure to do them. It can be kind of easy to forget in the midst of writing a blog post. doing a new web page or email campaign. But, remember that all the work your doing is not as strong with out the images being tagged.