Solution below is for D6 if you want this for D7 then you need to use https://www.drupal.org/project/tagclouds
You need to have taxonomy module enabled.
If not do it from modules page. Then click on Add Vocabulary tab and fill in all details, select content type for which you would like to add tags.
Adding this vocabulary will allow you to add tags while creating content of selected content type.
Now you are done with tagging content, This means whenever you go to create content of the content type for which you have added vocabulary you will find tags section on the form, e.g If you have selected content type ‘page’ to add vocabulary you will find ‘tags’ field while creating a page. this is a comma separated list of tags describing content of that page.
Next step is to install Tagadelic module for tags cloud
Download module Tagadelic for your Drupal version and enable it.
Go to this modules configuration page and make settings as you want.
you can do it under this page /admin/settings/tagadelic
Now you are almost done with it.
This module creates tags cloud block for you.
go to Blocks and search for the newly created block and configure it show in any region you want.
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