I'm currently low Diamond on EUW, with Janna as my most played champ.

Janna is one of the best protection supports in the game, which means her only job is to keep the ADC alive. She is best with hyper carries like Jinx or Vayne who deal insane amounts of damage when they live long enough. But she can be picked in almost every situation. You should only be careful to pick her when the enemy team has hard engage like Malphite because you can't really do anything against that type of champs.

Flash is necessary and cannot be replaced in the current meta. Exhaust is the second summoner spell I pick in 98% of all games because it is so useful to peel for your ADC. In 2vs2 fights on bot lane you usually want to use this spell on the enemy ADC. You can also use it to prevent a gank from the enemy jungler, or to set up a kill if your jungler comes to gank your lane. In mid/late game, use it on the first enemy that comes too close to your ADC.

The other option is Ignite which you can take against heal champs like Soraka or Swain, but I would only recommend taking it if there is no enemy champ that can be dangerous for your ADC, who you would need Exhaust for.

You go heavy on AP because Janna scales so good with AP. You could also go for Magic Resistance Glyphs in matchups against Zyra or Vel'Koz where lots of magic damage comes from the enemy support.
In the matchup against Soraka you can also go for gold per 10 seconds quints and seals, but that's not necessary and I would not waste a rune page on that.

I usually decide between Ancient Coin and Spellthief's Edge in the loading screen. The decision depends on the enemy team comp. If they have champions like Olaf or Fiora where your ADC is still not safe after you have pushed them back with Monsoon you should take Ancient Coin and build it to talisman of ascension and activate it in team fights, so your ADC can kite easier if they focus him too hard.
In every other situation you usually want to go for Spellthief's Edge. When doing so you have two options. You can go for Frost Queen's Claim or Eye of the Watchers. I only go for Frost Queen's Claim if I need the active to ward safely, if they have champs like Rengar, who one shot me if I face check a bush, I use the active first before I face check a bush in a area where he could be. The rule of thumb is to get this item whenever you play against champs who kill you often while you are warding. But usually I go for Eye of the Watchers because it allows me to build other items faster since it is cheaper than Frost Queen's Claim.

Early Game Core

Sightstone get ASAP since it is the most important item for supporters

Sweeping Lens once you have got your Sightstone you don't need your Stealth Ward anymore

Boots gives you more mobility and allows you to ward deeper into enemy territory without getting caught

Vision Ward always have one or two of them with you, especially when you play against invisible champions like Rengar or Akali
When you have decided to start with Ancient Coin, Nomad's Medallion is also core because it increases your income

These are the items you need before you start building anything else. Your future build decisions depends on the urgency of the mid game items. If one of the Items in the list of mid game items looks perfect for your current situation, then get this one now. If none of the mid game items is perfect then go for the other early game stuff items.

Other Early Game Stuff

Eye of the Watchers this item is quite stat efficient and the additional ward is nice (only buy if you started with Spellthief's Edge)

Ionian Boots of Lucidity gives you even more mobility to roam and ward

talisman of ascension the active is really nice but most of the time you won't need it that much in early game

Frost Queen's Claim the active can be quite good to create some picks onto enemy champions

Ruby Sightstone if you have decided to go for either talisman of ascension or Frost Queen's Claim you might want to build that one too if you have got nothing else to build, the 20% item active cooldown synergizes really well with both Frost Queen's Claim and talisman of ascension

If you win your lane take Dark Seal, if you also win the mid game then build it into Mejai's Soulstealer if not you can sell it when you need the slot.

Mid Game

always change your Sweeping Lens for Oracle Lens once you are lvl 9 or higher and you are in shop

Locket of the Iron Solari If the enemy team has three or more champs that deal mostly AP damage you have to get this item ASAP. I also often get this if the enemy has only two AP champs, but only of the enemy team has lots of area damage of any source because then the active is so valuable, I almost get this item almost every game

Banner of Command if they have lots of AP damage but not so much area damage, this could also be a good pick especially when they have only AP wave clear, since the minion you buff with the active of Banner of Command is immune to magic damage. You can also use this item to make a wave pushing by it self.

Mikael's Blessing I take this item often when I don´t know what to build since it increases your shield strength and the active is quite good to peel for your ADC. I often take this item first when the enemy have predictable CC like Death Sentence or Dark Binding. I call it predictable CC because you can already see almost half of a second before your ADC gets hit by it and you will have an easy time to remove the CC immediately and save your ADC's life.

Frozen Heart this is the best item if the enemy has a lot of ad damage or auto attack reliant champs like Kayle where 15% less attack speed equals 15% less damage

Ardent Censer take this item when you have a team where you would build frozen heart if you were the enemy

Zeke's Harbinger I don't like this item in the current meta because ADC's can build crit really efficient right now and the other ADC's who do not auto attack so much are not need crit at all. But you can build it if your ADC got really fed in early game and has rushed Infinity Edge because this combination can be really broken.

Rabadon's Deathcap I really like this item when you are ahead since it synergizes well with Mejai's Soulstealer which is another item you buy when you are ahead, and Janna scales so good with AP

Banshee's Veil get this if the enemy have lots of AP damage or the have an assassin like LeBlanc who likes to focus you, or they have CC like Death Sentence or Dark Binding. that will get you killed once you got hit and you are bad at dodging

Late Game

If you are full build can think about replacing your Ionian Boots of Lucidity with Rabadon's Deathcap to help your teammates surrive better or Guardian Angel to survive better yourself.

Elixir of Sorcery take this if you are full build

Elixir of Iron take this if you need the additional health and the tenacity however I almost always take the Elixir of Sorcery

Passive - Tailwind
that your ADC is in range to get the bonus movement speed when you are walking to lane together

Q - Howling Gale
Due to it's slow travel speed, this spell can almost only be used effectively on enemies that are standing still or moving towards you. If used correct, this spell can cancel almost all dashes and jumps except for unstoppable spells like Unstoppable Force or Cease and Desist. Don't just randomly throw this spell out in teamfight, only use it on enemies moving towards you and your ADC, and don´t charge it in teamfight, the only exception is if you are in a bottleneck area where the terrain prevents the enemies from dodging it. Sometimes you have to spare this spell for a specific enemy spell for example if the enemies have Katarina and you have no Monsoon then you can interrupt Death Lotus with Howling Gale.

W - Zephyr
You can use this spell to slow enemies and setup kills for your teammates or peel for your ADC. You can also use this spell to poke in lane. But be careful with the usage of this spell, because you lose the increased movement speed as long as this spell is on cooldown. In teamfights you use this spell whenever it is ready.

E - Eye of the Storm
This is the most essential spell of Janna. Train your reactions until you are able to shield every source of damage on your ADC in lane. Keep in mind that you also can use it on yourself if the enemy decides to focus you. In teamfights you use this spell whenever it is ready. You can also use the bonus AD of the shield to push towers faster. When pushing towers in group, use the shield either on the target with the highest attack speed to get the maximum damage out of it, or on a melee target to protect him from poke so you can safely take the tower.

R - Monsoon
This spell can be really strong when used correctly but can do great damage to your team if used wrong. The spell has two parts. First it knocks all enemy targets away, then it heals all allied targets. The spell can also be used to cancel abilities that deal damage over a period of time like Death Lotus or Life Form Disintegration Ray. You can also cancel dashes or jumps with this ability but there are some abilities like Elastic Slingshot that can be cancelled with Howling Gale but not with Monsoon. In teamfight I usually use the skill to either knock enemies away from my ADC or any other target I want to protect, or later in the teamfight I also use it to heal if a majority of my teammates is already wounded. The problem with this spell can be, when you have lots of melee champs, but the enemy has lots of ranged champs, and you knock them away, then your melee champs have to use gap closers to get back to the enemies or the get kited and that's why this spell can be really bad if used wrong. There are also situations where you can use this spell out of combat when you just want to heal some of your wounded teammates to siege a tower or start baron. Also keep in mind that this spell also heals your minions which can make it even more efficient to heal your teammates to siege a tower. But only do that if you are sure that you don't need it to peel for your ADC.