To do this requires some knowledge of HTML. This is a how to article for Google Blogger Bloggers on how to install a tag cloud on their blog.
A tag cloud or word cloud (or weighted list in visual design) is a visual depiction of user-generated tags, or simply the word content of a site, used typically to describe the content of web sites. Tags are usually single words and are typically listed alphabetically, and the importance of a tag is shown with font size or color. Thus both finding a tag by alphabet and by popularity is possible. The tags are usually hyperlinks that lead to a collection of items that are associated with a tag.
1.
Step 1

Tags for Google Blogger blogs are called "labels". The area to create labels is located at the bottom of new posts. You input the labels you would like to create in the labels text box and web searchers can locate your blog using the tags as search terms. In this tutorial we are going to create a tag cloud using the search terms so that users can access all of the posts associated with each tag.
2.
Step 2

Find an appropriate tag cloud provider for your blog. You can find animated ones, Technorati provides a good one, you can get paid for Zoomtags and there are a lot of others that you may use. I will provide some sources at the bottom. You can skip to the bottom if you would like to use any of the preceding methods. I used the tag cloud found at blog:http://blog.frivolousmotion.com/2007/05/how-to-make-awesome-tag-cloud.html. Just follow all of the instructions step by step there.
3.
Step 3

Preview your blog. If you have done everything right, you'll have an awesome tag cloud in the sidebar. You can move the widget wherever you like, and modify it if you really know HTML.