When you're writing a post, you'll have a space at the bottom of the form marked "Labels for this post." Then, click the "show all" link to display a list of labels you've used previously. Then just click on the labels to add them.
When you publish your post, the labels will be listed with it. Clicking any of the labels will take you to a page containing only posts with that label.
When you search for a specific topic later, you can select labels from here: