when you make a post, there is a box on the right that says tags, type in your desired tag and press enter to add tags eg:

the link to tagged post is http://YOURTUMBLRUSERNAMETHING.tumblr.com/tagged/TAGNAME.

so for example, for me to add a link to all the posts that are tagged as “tutorial” on this blog this would be my code:

<a href=”http://milkxotheme.tumblr.com/tagged/tutorial”>tutorials</a>

for more information on adding links see here http://milkxothemes.tumblr.com/post/1085420408/making-a-link