You need to find tages that are related to your keyword..use 7 to 10 related tags for your post and tag it..
You need to choose 4 to 6 tags from your content but make sure one of them must be subject relevant tag.
Try thinking like the people you want visiting your site. What would they be doing and how can you make them stay there. Also find tags related to the theme of your site but make sure that the visitors don't get distracted.