This is now finalized.

Soraka is what I would call a long distance tank. She can tank better than anyone else in a long term fight, but less so in a burst damage fight. The key to this build is using the Alt key for self heals (I have healed/silenced minions so many times), and just about constantly spamming starcall.

The idea of this build is to capitalize on Soraka's starcall rather than her heals. The idea is to always be in the middle, or close to, and spamming starcall.

When tanking as Soraka you want things that give you armour and magic resist. Things like warmogs and leviathan only decrease the value of your self heals.

Pros:

-really complements magic damage carries
-is unexpected
-taking down tanks 1v1 as a healer is a great feeling :P
-you make a great distraction: they always target "squishy" Soraka first
-you can lane forever

Cons:

-your team will ***** about you not healing them enough/powerfully enough
-they will want you to go ap/support (boring!)
-you can lane too long, don't forget to buy items
-you potentially push a lane too hard with starcall

The build order I have given is very basic. In any situation I will start with the mana and two potions, but from there you must CUSTOMIZE the order you buy in based on who you are against. Every time you die, check the death recap and see the ratio of physical to magical damage. Then buy items from your recipes that will counter that: health, magic resist, or armour, accordingly. And if you are rocking the house, go for offensive items first like the scepters and sunfire.

1) Boots of swiftness: you need this for running away while starcalling as well as chasing while starcalling. Doing these both is what you need to do in a 1v1 fight. Chase them, let them chase you, go back and forth while starcalling to get your stacks up, then once they're high enough and low enough life you can tank them.

2) Spirit Visage: this gives you some health, magic resist, and cooldown reduction, but most importantly increases your self heals, which you will be spamming.

3) Abyssal Scepter: this complements your starcall and is an obvious choice. Your purpose in life is taking down magic resist. It also increases your magic resist.

4) Crystal Scepter: this makes you beefier, and also gives you two slows. This, with your boots, means that nobody escapes Tankraka! Chase down the weaklings while spamming starcall and they will die. Your silence will also slow them more than your starcall (as it targets only one), and has a slightly larger range.

5) Frozen Heart: this increases mana, cooldown reduction, and armour. Furthermore, it slows down enemies' attacks, which is a good thing because you are always in the middle of fights.

6) Sunfire Cape: increases armour which is nice, but more importantly adds magic damage per second, which gets increasingly strong as your starcall stacks add up.

If you can lane with someone who doesn't need mana, you will do much better.
If you can lane with someone who does magic damage, you will do much better.
Therefore, lane with kennen XD

If you can, zone control. People can't really harass you early game because you are always healing. This is especially effective in a 2v1 lane (where you are the 2). You let your ally last hit while you stay zoning the enemy by his tower.

Use your ult to ****block Karthas as much as possible. Try not to waste it on lost causes. Keep an eye on your mini map, especially early game, and you can really make a difference for other lanes: either saving them or allowing them to actually get kills in close fights.

Another great use of your ult is sniping gold. I advise practicing this against bots first. Even if your ally is at full health (and more so if he isn't), and is about to get a kill across the map from you, use your ult and you will get an assist worth approximately $150. This is nice, especially with your cooldown reduction. Since Tankraka can effectively jungle at higher levels (and should be taking blue often), while u stand by the golem or dragon casting starcall, just watch allies' fights and save them/snipe gold :)

If playing against invisible *****es like teemo eve etc, you will find that starcall works like a radar. If you can cast it, it means there is someone in the radius. A few times I did this and by the time eve stunned me she was basically dead. CAUTION: against teemo, he can't move, so you can get him very easily, but you can also cast it on his mushrooms, so don't waste your mana until you know it's him.

Exhaust takes down their magic resist, slows them down, and stops them from killing you. All of these are great for Soraka. This can often get you first blood with the right laning partner.

Heal increases your use, your tankiness, and saves you mana from having to heal yourself and others more, leaving more mana for killing time.

The idea is you max starcall as soon as possible. Sometimes, however, you have to sacrifice a point in starcall if you are really mana or health hungry. Adapt which of the heals you increas based on how the match is going. If you and your ally are at perfect health, don't upgrade heal before silence.

Mana regen means you need less (no) points in utility, and less items in regen.

Cooldown reduction is necessary because you will not always be able to get your cooldown items first. Even at your end build you have only about 30% cdr from items.

A little bit of magic penetration is nice to beef up your output, especially early game, but maxing it out does not help much because you are hopefully taking down enemy magic resist a lot already.