I know I can do this, but I just can't find the button...
How do I follow a tag?
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Depends on what you mean by 'follow', but you can hover over the tag to get access to the subscribe and rss links, and you can also use favorites (accessible on the sidebar) to generate a collection of 'interesting' tags. You can also use the ignored feature to generate a collection of tags that you're not interested in (so they don't show, or at least don't show as prominently on the site for you.)
Thanks to Bill's note I just learned you can use the star in the tag-hover-popup to toggle your preference between favorite, ignored and normal (saving you from having to use the side forms).
Your second (now gone) question is a completely different question, and besides not being appropriate having two unrelated questions in one, the topic has been previously covered, should you care to search.