If you really want to be a great support, you first need to stop caring so much about yourself. Sure you need to keep yourself alive, but always keep your eyes on other people, making sure they have enough health and mana, shield them if they're being attacked or running into a fight, use a tornado to knock back enemies, and ALWAYS ALWAYS ALWAYS stay near them, you never know when you might need to Monsoon to save someone. Having a good support makes all of the champs on the team perform better. You can push farther, die less, kill more, it just makes for an overall better team. Another thing about Janna regarding farming -- you can use your Howling Gale to get minions to half to low health and let your ally/lane-partner kill them to get the gold and extra experience. Although you do need gold, be sure you let your teammate get all of the last hits. And, for killing champs, it's usually best to do some sort of combo. Use Howling Gale as an instigator, to knock back your enemies and slightly lower their health. Now, as your ally is running in, use Eye of the Storm to shield them and boost their Attack Damage and Attack Speed. Then, use Zephyr on the enemy to damage and slow them. Finally, just sit back, keep shielding your ally, and watch them score the kill. About your ultimate, I think it's one of the best in the game. For those of you new to Janna, Monsoon blows away enemies and allies, restoring 150 (plus 35% of her ability power) in permanent health. It's perfect for escaping from team fights, even if you are fine, let's say two or three of your teammates have dangerously low health and are being attacked. Place yourself between your allies and enemies so that when you Monsoon, your allies will be healed and blown farther towards your base, while the enemies will be blown away from you, keeping your teammates safe and preventing feeding.
Janna is my main, and my FAVORITE League of Legends champion, I hope this build helps and you love her as much as I do! Have fun guys!