You can now search for an interest, hobby, or community in the same way you'd search for a user — just type it in the search bar and click through to the hashtag's landing page. Here you'll have the ability to follow (or unfollow) the hashtag in question, as well as explore related hashtags that might also spark your interest.
As with standard searches, you will see the top posts on the top and the most recent posts on the bottom — so be sure to check both places before you make a final decision on whether or not you want to follow a hashtag!