Well, this is my first Build, so, I beg your pardon for any mistakes I had done. This Build had fit well in my games, so I've wanted to share it. My choices are based in this think: To make a good build we need to upgrade the natural attributes of the champion in order to minimize the problems that it's worsts attributes will bring. Look, it's useless (or almost) doing an APC Yasuo, or an full tank Syndra, so we need to fit the build in the champion's natures.
As you know, Janna's best attribute is the movement speed and, it's ult becames a powerfull heal when we have some Abilitty power, in other case, it just serves to controll the lane throwing enemies in the direction you choose (but, to make that you need flash or a good buff in speed), as the ultimate, the shield of her E buttom it's a good way to take your ADC to home safe, but to make that, you'll need Abillity power.

Well, Flash is a great choice to early game escapes, and to help you to position yourself in a Team Fight, why? 'cause you can flash behind your enemies adc and throw it (with your R) right in the mid of your team's place. And, Exhaust its great to save your ADC in any part of the game (early, mid or late) or to slow your enemies ADC disabling him tries to escape. Try to use Exhaust and W together or one after another.

This items build foccus at Movement Speed and Abilitty due the reasons I've put in the Introduction but, let's see better why do we choose that items, one by one.
At the start, as you're a Support and don't farm, you need an gold item, and, we choose the coin because, at it's last upgrade, the Active power its very usefull; The ward it's a basic need of the bottom lane, as the others, that's why we need it, and the Trinket. But, keep in mind that that trinket it's not the better choice to the mid-late game due your Sightstone.
At the core items we have a Mana item, wich is essential due the expensive mana cost of her Q. Keep in mind that you can change the Tear, as I've told earlier, but, if isn't the case that your adc really need Mikael's ative power (if you're fighting with a Malphite, Leona, Fiddlesticks, Annie, or any other character that have disabling powers, and, you see that your ADC haven't enought escape abilitties you'll need to change to the Chalice, but I dont recommend), the Tear's will be one of your principal sustain item with its mana shield in Seraph's embrace form at the late game, with this you can position yourself (without so much fear of dying) to use the R as I've especified at the 2nd Chapter.
Your needing of Ore just get greater, so, thats why we get Nomad's Medallion. And, the tome starts to build up your damage;
As you are in the mid time of the game, its usefull to have the Active's (and Passive's) abillities of the Talisman, and, of course, the Twin Shadows, wich is great to force a Team Fight start or to prevent your enemies running at it's end. The boot is a good increase at your speed, our best state. As you're advanced in the game you'll probably need more damage, that's the time of the Damage Stance: Finish the staff wich'll provide a good amount of Abillity power (you're a support, keep that in mind) and Lich Bane will both increase your Abillity-damage given and speed; it's passive fits well after using W.
Upgrading your boots will let you almost unachievable, but we're trying to get at the Unachievable (what is almost impossible but...).
Now, at the end of the game, probably when someone got the baron buff (your team, or not...) and when one team its at the Inibitor's towers of the another (your ones, or not...) you won't need that much of warding (if you need, change the Trinket again) so, try to increase your damage (Abillity power) and Speed selling the Sighstone (that you shouldn't have upgraded) and buying Trinity Force, keep in mind that is better to start buing the blade, that'll build up with Seraph's Embrace, that's you supposed to have at this point. To the Gran-finalle try getting the Enchantment (you can get it earlier) and finish your trinket. The Red one its not Imprescindible, you can choose between it, the Blue, or, at last instance, the yellow.

Well, thank you for reading, I'm probably upgrading this aesthetically and with some tips, but, playing with Janna isn't hard. Have fun, try to don't let your ADC dying, don't KS him (try it :D), and don't get angry. She is an offensive stance support, and to that you'll need make good use of the signals, and, maybe, a little of harmony with your ADC. I hope you liked. TY For Reading and cya at Lol :D