If you find a user whose work you're interested in, you can follow either from their profile or the gallery.
Behance Galleries: If you're clicking through projects, you can follow members straight from a project. This way, you don't have to interrupt your browsing at all. Just click "FOLLOW" on the right hand column that includes all the project information.
Behance Profile: Click "FOLLOW" the right of his or her profile photo.
To Unfollow someone you're following, visit their profile and click "unfollow." Or, click on the followers section when viewing your own profile, find the person you'd like to unfollow, and click "unfollow."