Use these square brackets "[]"s with the words you want inside it in your thread title. For instance, to have an open tag, you can have "[Open]". For a private tag, you can use "[Private]" or "[PV]" and so on. You can find out more information here.

Hi Maximus! To add a tag (I'm assuming you mean the ones in the titles of topics) you just need to write out whatever you want to be in the tag between brackets in the title area. Hopefully this makes sense. . . if not, there's a screenshot below.

EDIT: Oops, Hjørdis answered before me. Oh well. The screenshot may be helpful though. . .