Start item:

Your normal start item

You might want to start with this item to make a , I don't recommend since it doesn't give armor anymore. Dr Mundo starts with this item to make a spirit visage with it.

needed for Singed: he can use the mana for extra fling at the beginning

Boots:

Best tank boots: they give MR and the reduce the duration of stuns, slows, taunts, ...

If the other team doesn't have a lot of AP or cc

Usefull for singed to get his gases in the face of enemy and to easily fling them.
DONT buy this with rammus: you have powerball to travel fast

Items to continue the builds:

If you're not taking to much damage AND if no one else is going to buy aegis of the legion

If you take a lot of physical damage or there is a xin/trynd/yi/noct/...(huge physical damage) in the other team

If you take a lot of magic damage (you can stack this item for loads of MR and a lot of movement speed)

If you need more armor after you bought Thornmail

If you need more MR after you bought Force of Nature, although i would stack force of nature in that case

For Dr Mundo: if everything goes well with 1 warmog, a second one is a good idea

A good item to finish your build, I mostly buy this if I really don't know what else I should buy, it gives a nice amount of armor and MR.

Just put most in defense...

I bought all defense runes; some for health, some for armor, some for MR
they are all /lvl because those give the most at lvl 18

Start with 1 lvl in for traveling, slowing and harassing
Get puncturing taunt to lvl 5 as fast as possible and after this get to lvl 5
lvl 1-6
It is possible to get the first kill by slowing the enemy champs with powerball and some help from your teammate
lvl 6-12
Try to stay close to your turret
With puncturing taunt you want to hold your enemy within turret range
If you push to the other turret use and Tremmors on the turret to make if fall fast
lvl 12-...
In teamfight try to taunt the AD champ, if you have he should die pretty fast

First get to lvl 5
the is a useful finisher, you can also heal yourself with it if the enemy aren't too offensive.
lvl 1-6
Harass and/or kill your enemy's by spamming
lvl 6-12
keep spamming the and look out for friendly champs with low health to use on.
also help your lanemate bye trowing your at the minions he is attacking
lvl 12-...
in teamfight use your and so your team stays safe

First get to lvl 5, its useful to damage and slow enemy's
lvl 1-6
Spam on your enemy's, if your mana gets low you have Clarity
lvl 6-12
Keep spamming on your enemy's, if they attack you or try to run use on your enemy's, combined with , and some attacks of you lanemate the enemy champ should be pretty dead
lvl 12-...
keep using the / / combo in teamfights and you should do OK

Start with 1 lvl in , after then get to lvl 5, you might want to get 1 lvl in to slow your enemy's more
lvl 1-6
try to your enemy's into your tower, don't use yet since it uses to much mana early game
lvl 6-12
use only for attacking champs, try to them into you poison
use for hitting the turrets harder
lvl 12-...
once you bought the 3 starter items you should have enough mana/mana regen to leave your on all the time. still try to champs into it, try to run in front of enemy champs to give them damage of you
if you need to flee, use and Ghost; put on and maybe slow them extra with

It is possible to mid with Dr Mundo: his cleaver and his healing will keep him alive and may get him a kill

Get to lvl 5 first
lvl 1-6
Spam your cleaver to everything that moves
If you are low on health you might get 1 step back and wait for you passive to regain health, if you get to lvl 6 you can heal with
lvl 6-12
just keep spamming cleaver, use on turrets for loads of damage
lvl 12-...
Do a lot of damage in teamfight and flee with after some seconds your healt should be high enough to do a second attack

• Take most of the damage; you can take it, your teammates can't
• Save your teammates even if it costs you your life: you don't want the enemy's to get +1000G for stopping a godlike, a tank normally doesn't have a godlike they wouldn't get +1000G
• Taunt, slow or exhaust the enemy's so your team can catch up with them
• Flash away if Tryndamere uses his ultimate! He has 5 seconds to kill you (5 * 2 hits/s? * 1000damage? = to much for anyone to handle)

You will get depressed of all the killsteals your teammates do on you
You will get depressed if Tryndamere keeps using his ultimate

Life of a tank is hard...