First of all, tanking Janna is kinda fun.
Your Janna will become a pretty good tank and still be a support/sitter champ.

At the beginning start with "Regrowth Pendant" and make it ready with the "Catalyst the Protector" cause you immense need of mana for your job as supporter/sitter.

Next buy yourselve the "Mercury's Treads" for your movementspeed and magic resistance.

Now you start tanking Janna.
-Begin with "Warmog's Amor" for huge Healthpoints and regen.
-Go on with "Thornmail" which is pretty fun cause you gain +100 armor and it returns 30% of the damage.
-As next get the "Force of Nature" for high Health regen..
-Now "Guaridian Angle". (amor/magic resist and revives your champion upon death what sometimes saves your ***)

I think i dont have to tell much about the runes

- Greater Mark for your amor
- Greater Seal for your health
- Greater Glyph for your magic resist.
- Greater Quint for your health regen.

Your Masterty is focused on "Defense". That's why i choose 27 points here.
But i would also set 3 points on "Perseverence" for a greater health and mana regen.

Here I'd choose "Heal" and "Flash".

-Heal complements pretty god to your job as the sitter and could save your's and allied lives.
-Flash's good for saving your own ***^^

alternativs here:
-Clairvoylence
-Clarity
-Teleport