Hi LarVak and welcome :)
If you want to become someones fan click on their name, which will take you tot heir profile page. Then on the right just below their picture there is a link which says "Join (Hubbers Name) Fan Club. When you've done that it then offers you the option to post fan mail to their profile. To receive your fan mail go to your profile and check down the right hand side of the page - it will say if anyones fanned you and give you links to see the fan mail.
To reply to comments simply post a comment in reply to either on your hub or someone elses. The comment boxes are usually at the end of the hubs.