AA then Alt+E(explained in TIPS chapter) is a great harass tool early on. Try and force trades but be carefully vs all in comps like Grave and Leo. Shielding is the key to winning any lane with Janna. Shield should be used to prevent damage as well as increase damage of your ADC's spells and attacks. This is why Janna is amazing at trading in the lane. She prevents enemy damage and increase your own damage with one spell.

Janna is very fast all the time. This is great for warding safely. You can dodge many skill shots with ease and get in and out very quickly. And if you have ever tried to chase a good Janna i am sure you found it a daunting task. He kit is the best disengage there is besides maybe a max range lantern from thresh. Janna can take towers very quickly with an ADC like Tristana or Jinx. Her shield gives more ad than a bf sword late game. Add the attack speed bonus of Ardent Censer and your marksman is now a tower taking machine. And with the disengage Janna's kit allows you can push these towers with little fear of being punished.

When taking Dragon or Baron it is important to position in a way that the enemy jungler has to go through you. Your Q and R are amazing tools for pushing any chance at a smite steal away. Also your shield should be the champion tanking early game and on your ADC late game for damage.

In my honest opinion the two best team fighting supports are Janna and Lulu. Janna has the ability to completely stop a fight with the press of her ultimate. This allows your team to reorganize and prepare for an engage at a later time that better suits your terms. There are a few ways I like to use Janna ultimate based on the situation.

1. Stop the Fight: By blowing your Q and R quickly and then popping your Talisman of Ascension your entire team can escape and regroup. I use this method when my team has a poor engage, when the enemy team engages at a bad time and we don't want to fight(low health or waiting on important item/cool downs), and when a player gets picked off and you need to cut your loses or get aced.

2. Counter Engage: Few champions can stop and ruin a hard multi champion engagement like Janna can. If you can ruin the enemy engagement, and they have blown several of there key engagement tools(such as ultimates/summoner spells/Righteous Glory), you and your team can turn the tables and fight a team that no longer wants to. (An amazing example of this style of play is here) Get them to blow there engage tools and then turn the tides.

3. Split the team: If you can ultimate in a way that you send most of their players flying away from you and 1 or 2 of then are sent into the middle of your 4 other team members. Unless they are very fed tanks they will die very quickly. This method allows for your team to gain a numbers advantage and often forces the other team to cut there losses and back off while the victims die.

Your main goal however is not to single handedly win a fight with your ultimate.(Though it will often be the case) Your main goal is keeping any threats off of your carry so they can maximize damage with minimal damage taken. Can't stress enough how many games I kept the hyper carry Jinx alive and watched as she rips through the team with no other major assistance in damage. Play this role enough and you will get a feel of who to exhaust and who to peel. When to shield a front line tank or the ADC. And honestly when you do everything perfect, utilize every resource your champion has, and your team stands after a team fight in victory all with minimal health bars, the feeling is amazing.

Q can be used to stop nearly any dash. Though you can't stop instant dashes like Shun Po or Malphite's Unstoppable Force, you can make it very annoying to engage on you or allies. Leblanc tries to W to your Katarina who is waiting for a good time to join the fight and reset. You saw her coming and charged your Q. The tornado stops her dash a couple steps from where she originated. Now she has to blow her Mimic just to get into range of your carry. She dashes again and you send her flying back mid dash with a quick ultimate. You have just made their assassin worthless while the enemy team is worn down and then demolished by your untouched Katarina. Be aware of who is targeting your friends and cut of there path with a winding pillar of wind.

A cool trick with Janna's shield is that it applies damage to projectiles fired before it is applied to the champion. If Ezreal fires his Q and Janna shield is applied to him after he cast the spell but before it hits the target the bonus AD is still applied. This also applies to auto attacks. If you have a jinx that fires a cross map ultimate make sure to shield here right before the rocket hits not when she first casts it. Otherwise the shield could time out before the rocket hits it's target. Also Janna can get assists very easily by sheilding allies(this is why she is so good with Mejai's).

Janna ultimate heals a lot but most of the time you will ultimate and have to cancel it very quickly as you were only utilizing the knock back.

Janna shield is the only one that can shield towers from damage.