It's going to depend on your website platform. If you're using Blogger, there's a section on the bottom of the post called Labels. Just add keywords relevant to the blog post you wrote.
If you're using Wordpress, they have an actual section called tags in the side bar. You do the same thing.
In Hubpages, they suggest tags for you to use. Try to also add tags that makes your Hub show up with other hubs talking about a related topic. Helps give you more exposure.