A very simple method to add the tags on to a wordpress theme.Its pretty simple just add the below lines to the file you want it to appear, prefferably SIDEBAR.PHP.This can be easily done using  wp_tag_cloud(), 

Here the smallest means the FONT size of the catagory where least number of post have been made,Largest means the the FONT size of  catogory where the most number of posts have been made.     

<?php wp_tag_cloud('smallest=8&largest=36&'); ?>

 

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