Welcome to Hubpages! There is no chat here in HP.. the closest thing is the forums for interactions with other hubbers. I hope I knew about the forums when I was new.. I'm still fairly new, only been here six months. In the forums, you can ask for help and advice from other hubbers, you can have your hubs critiqued if you want other hubbers to help you improve your hubs (one at a time tho'), you can join challenges like the 30hubs in 30days, or play word games with others in the sandpit.
To answer your original question on how to leave a fan mail - You follow another hubber by going to the profile page of the hubber and clicking "follow (hubber name)" located below the profile picture. Right after you'd clicked the follow button, the next page that appears will say you might want to leave the hubber you just followed a fan mail and space will be provided on that page for fan mail. You might have to scroll down the page sometimes.
Best of luck here in Hubpages. This is a really good site.