Heya!
If you mean forum tags, heres how to add them:
Navigate to AdminCP > Forums > Forum Tags (tab)
Click New Tag > Name it > Set the forum you want it usable in > Set the style.
Styles are the coloured tag previews below the Name input box
Finally Click Save > Go to the forum that you added the tag to > Test it with a forum post.
Hope that helps, if you don't understand just say below and I'll try to explain in a video :D
- Tony