Please note that this is my first time I'm making a build here on Mobafire.

Now let's go on with the build ^^

First off I grab a Cloth Armor and 5 Health Pots.
After laning and stuff, go back and buy a Chain West.

After additionaly laning, go back and buy Thornmail.
After the Thornmail I recommend Boots of Swiftness, because I think tanks need to be fast, to save people, engage, get in fast and get out fast out of battles etc.
The good thing about Amumu is that even if you go tank, you can still make huge damage.

Aight.. after the Boots of Swiftness, I usually go for Force of Nature.
Then go for the Aegis of the Legion, 'cuz it gives you a good amount of Armor and Magic Resist, + it gives you health.

After the Aegis of the Legion, go for like 2 Warmogs.
Also, another alternative is to buy 1 Warmogs + Guardian Angel.

PLEASE note that this is my first build, EVER, to publish onto Mobafire.