The simple way would be to install All in One Seo Plug-in and go to settings, then simply tick two categories that are there for using nofollow with tags and Categories. This is the fastest way that comes to mind.
You can use wordpress plugins which you have to activate after transfering it to plugin directory. After activation you'll get an extra option which will be seen in cat option,there you do nofollow. The another method is to us the following tag:- <a rel="nofollow" href="yourdomain.com">tag</a>