Despite what many people think Janna is a support and attack champion. There are two effective ways you can build Janna; attack speed/attack damage and AP/cooldowns. This guild will discuss the way I play Janna as an AP champion. Janna is a powerful ally and can be a powerful enemy. You must remember to play it safe at the start if you are against aggressive or strong opponents.

For my summoner abilities I generally choose Clarity and Heal. This allows me to stay in the lane longer at the beginning and can also assist the ally I am laning with. 9 points in offense masteries allows me to get the magic penetration point which I find helpful against high magic resist enemies. Have a lower cooldown on the heal is also good if that is one of the summoner abilities that you choose. Extra gold and experience are a good help for any champion.

The items I get as any champion generally depends on how I think the game is going. I always start with:
1. 'boots of speed', 2 health potions and a mana potion.
-I tend to use the spell clarity for mana first if i need it as I can have the cooldown going early (and use the potion if the clarity cooldown is still a minute or so away and I need mana). The 2 health potions are more so a precaution as I never know if the enemies I am laning against are aggressive or not.
2. 'Rod of Ages'
- Since this gives you mana and health I like it for the survivability. It also gives a good amount of ability power. I try to get this at the start so that I get the maximum cap bonus early as possible (+20 AP, +180 health and +200 mana).
Somewhere in here I also purchase 'sorcerers boots' for magic penetration.
3. 'Nashor's tooth'
- Next I aim to get the cooldowns low so that you can use her abilities as often as possible. Nashor's tooth is great as it gives her faster attack speed so that when her abilities are on low cooldown you have a faster attack as another means of fighting. We also cannot forget that this item also has AP as another positive.
4. 'Morello's Evil Tome'
- As soon as this item was created I thought it was great. It gives Janna additional lower cooldowns plus AP and mana regeneration.
*Note - I am aware that there is a maximum of 40% cooldown reduction. Therefore some of the cooldown from the items I purchased is not maximized. You can exchange this for 'deathfire grasp', but I tend not to as Morello's has a higher AP and mana regeneration. The active for deathfire grasp is useful but I personally forget to use the actives from my items.
5. 'Rabadon's Deathcap'
- This is normally my last two items. This is because it gives the most AP out of all the other items. The unique passive is not given twice but it is still the largest AP item (well unless you would like to use 'Mejai's Soulstealer', but this is based on stacks and I like to ensure I have the same cooldowns and AP regardless of how many kills/assists I get (more reliability).
+155 Ability Power UNIQUE Passive: Increases Ability Power by 30%

I used to play aggressively early on with Janna so I tended to level up her 'zephyr' to it's fullest first. I have weighed all the different choices through lots of experience playing Janna and now believe that 'howling gale' is the best spell to level up first to it's maximum. This allows you to gain experience and gold early on. I was finding that once everyone starts laning in teams it makes it hard for Janna to gain gold using her howling gale (as it took too long to charge and most of the creeps had been killed before it is released).
I always grab one level of every spell when I get to level 3 with Janna. This is because her spells have a high cooldown when you are in a direct fight. If you are with an aggressive ally in the lane, the 'eye of the storm' is a great way to both increase their attack damage and ensure their safety.
Janna is a great mider. Even if the other person in mid deals a lot more damage, Janna generally has the advantage with her great farming skills. Enemies tend to stay away from her howling gale which is great as it means less experience for them. If they don't stay away from this spell then it does quite a bit of damage to them.
Against other squishies like ashe, teemo and twitch, Janna is great against. Her howling gale means that at the start of the game the enemies take a lot of damage from the creep wave (this is generally the case in mid against ashe for example). They are forced back to the turret and the creeps can then deal the enemy turret damage... even if you are too cautious to go in.
If I am '1v1' with Janna and I know I can kill the opponent, I put eye of the storm on then use zephyr to slow and damage the enemy. The enemy generally attacks face on and doesn't move much which means you can charge a howling gale to it's maximum. Sometimes I like to start a howling gale and then slow the enemy with a zephyr (releasing the howling gale when they are slowed in it's line of damage).
Howling gale is a great escape spell for you and for your allies. It also interrupts a lot of ultimates that deal great damage to your team (save for when katarina is going to use her ult or nunu). It can also be used on fiddles when he is using his ult (it doesn't interrupt it but it stops him from harming your allies a bit longer).
Monsoon is great for turning the battle around, getting your allies out of tight jams and even can help you kill enemies if you use it right (knock them into your turret or separate one from their group.

Well I hope this helps some people out. I hope I didn't write too much :P
Here are some screenshots of score that I have got with similar builds
http://img808.imageshack.us/i/jannascore2png.png/
http://img848.imageshack.us/i/jannascore1.png/
http://img156.imageshack.us/i/jannascore2.png/
http://img43.imageshack.us/i/74656166.png/
http://img690.imageshack.us/i/jannascore3.png/