[2.0.0] Janna Guide - A casuals guide on how to climb as Janna

Starting Items
These are your standard starting items. I usually take [amplifying_tome] to boost my poke and shield a bit. If the enemy has bigger poke-range than you and you ADC, you can also pick [ruby_crystal] for a bit more HP.
I rarely get [boots_of_speed] when starting the game, but it can be quite fun if you know your teammates and manage to roam to midlane or enemy jungle together.

Core Items (Magic)
These choices are all AP-orientated and boost your DMG, but your shields (+ the AD it gives) and heals too! Generally you get a mix of magic and defense items, but if you really stomp a game... why not have some AP fun? :)

• Harmonic Echo Great item, lets your 3rd ability heal several teammates (and you). You should almost always get it, as item 1 - 4.
• Ardent Censer Also a very good item. gives your shielded teammate and you attack speed and bonus damage. The stats also work great on Janna. Can be acquired as first to 4th item.
• Rod of Ages A very good choice if you win lane and want to capitalize in form of some serious stat boosts. After 6 minutes it gives a toal of: 450 HP, 400 MP and 120 AP! And the passive also heals you a bit whenever you spend mana. Crazy good value! This should be either 1st or 2nd pick.
• Athene's Unholy Grail Also a good choice, especially if you have problems managing you mana or the enemy has good magic damage. It also grants a healing effect on your 3rd (and 4th) ability, if you poke your enemies.
• Morellonomicon Not your everyday choice, but it can really help whenever the enemy has some HUGE heals or sustaion. I'm talking Mundo, Olaf, Soraka, or even your fed enemy Draven with 30% Life Steal (tough, that will be a hard game to turn). Oh, it also boosts your HP, heals and poke too!
• Archangel's Staff Only used it a couple times, when it was pretty clear early on that we scale better or at least have some good chances to win. The more abilities you use, the more mana you get. At max capacity it turns into Seraph's Embrace and gives ehen more AP and s really good passive shield when you fall under 35% HP. Also, you won't have mana problems as much. In combination with other items and a high level it also gives a pretty good amount of AP (at least ~100AP)
• Not listed, but it goes without saying that you can always purchase a Rabadon's Deathcap to boost your damage and shield/heals!

Core Items (Defense)
Here I will talk about more defensive choices for your build. 

• Dead Man's Plate I recently picked this one up. It is crazy fun to just zoom around and poke the enemy even better! It's a great pick against AD damage (hard or mixed). It gives the basic stuff, good HP, good armor. But the real value is the fun (and effective, if positioning is well) passive ability!
• Iceborn Gauntlet Another good choice against AD damage, tough it gives MP and ability haste instead of HP (so less tanky overall than Dead Man's Plate !). It also gives a (really fun) passive: an aoe slow on your next basic attack after using an ability. Generally great to annoy your enemies, help a bit in teamfights, but also to boost your damage output.
• Zeke's Convergence Zeke's is a good choice for overall tankiness, as it gives armor and magic resist, but also mana and ability haste. The pasive on this one is generally great, but as a Janna you will rarely fully utilize it. Most of the time you will the positive effect to a teammate, but the slow and damage to an enemy with your blizzard will be nigh impossible, as you throw them away with your ultimate ability. Tough, it has some usefulness in teamfights!
• Protector's Vow A good item. It gives good HP, armor and ability haste. But the passive it what makes you buy this item. It protects you and your next allied champion and gives you a shield and movement speed if you get attacked by a champion or tower. Keep in mind that you have to stand relatively close to your desired ally and the passive has a 30sec cooldown!
• Abyssal Mask Another good choice against enemy AP damage. It gives HP and MP, magic resist and ability haste. The stats are already great. But it also gives two passives, one that heals you slightly when spending mana, but ALSO: it increases all magic damage dealt to nearby enemies by 15%! That means, that all your teammates do more magic damage to enemies when you're near the enemies. If your team relies heavily on magic damage, it can be a great pickup (equally for your defense, and your allies offense).
• Guardian Angel Now, don't laugh at me. The item is pretty much OP in this game, as the passive, that revives you, only has a 3 minute cooldown. If the game goes on a bit longer, it can be a great value, as it allows you multiple revives. Oh, it also boosts your AD (might as well take it) and good armor! If you're not hellbend on tryharding the game, it can be a very fun item ti pick. You certainly will get questioned, tough.

Boots of whatever you need
Now, you could basically pick every boots. Janna is very versatile in her build (as you might have guessed by now) and can utilize almost anything.
ALSO: don't forget to use the active ability of your boots to get in lane faster or chase/run away!

• Ionian Boots of Lucidity This will probably be your basic purchase. It gives ability haste (nice), decreases the CD of your summoner spells (Nice) and also increases your movement speed (NICE). All boots give the movement speed, tough (so, NICE).
• [mercury's threads] A great choice against an enemy team with high magic damage and/or much CC! The magic resist is nice to have, but no much. The tenaciry is probably more useful.
• [ninja tabi] Good item against AD heavy enemy teams, especially if they use a lot of auto attacks (marksmen/ADCs, but also fighters like Tryndamere, Master Yi, etc..
• Boots of Swiftness Even more movement speed! And reduces effectiveness of slows. If you want to go fast, get those sneakers! Keep in mind that they offer less stats that are useful in combat (armor, magic resist or ability haste).

Boots Enchantment
Your enchantments take the active of your boots away (=sad) but do give some pretty amazing actives in return!

• Shadows Enchant The enchant I used the most, I'd say. But only because I didn't dare to try the others. These are a great and especially easy choice of enchant, as all others recquire a bit of experience to use. I'd recommend this to all players learning the game still. It sends out 2 ghosts who track down enemy champs to slow them. Can be used to chase someone, to further slow in a fight, to excape, and probably to annoy your enemies even more. Cooldown is also quite low.
• [locket enchant] Good teamfight option. Gives a shield to you and nearby allies. The more, the merrier! Keep in mind, while it has a generous range, you should always aim for all your team members to get a shield!
• Redeeming Enchant Great pick for teamfights! It heals your allies and damages your enemies (with true damage!). Tough, it takes some practice to aim it. My first few redemptions didn't hit a single champion! I suggest using Co-op games or at least normals to get a feel for it. It is quite "wonky" to use.
• Glorious Enchant Does what it says. Gives bonus movement speed towards enemy champs and turrets, and slows nearby enemies. It can be useful, but it makes you run directly into enemies. You should be somewhat tanky if you want to pick this up!
• [stasis enchant] A somewhat selfish choice, but: keeping you alive helps your team out. You can use it to negate the damage from many ultimates or bursts (think Zed, Fizz, Lux, Vi, Darius, ...). You need to be able to rely on your team to guard you while you're in stasis. Otherwise, you will, most certailny, still die.
• Teleport Enchant Very versatile. Can be used to get somewhere quick. Either get to a teamfight faster or to an objective. I'd really recommend this to players who already have a good map awareness, so you can use it to its fullest.

Conclusion:

As you may have thought, it's best to know your items and react to the progress of the game (also true for any other champion/role).
If you're unsure what you should build in which situation, just take it easy and use the pre-made item set in the game. It's not bad. Try to get familiar of your choices, and try to pick them and use their effects to the fullest. Also, you can experiment! Just because I didn't mention an item, doesn't mean it can't be good on Janna. Who knows, maybe the next meta thing will be ADC Janna or something!
Also, try to have fun while paying. That's what keeps me playing (and climbing) soloqueue! :)