I made this build because Amumu is one of my favorite tanking champions. He is great for tanking if you know how to build him. This guide should help you be a better tank as Amumu.
He has some very good skills to help your team in a team fight which makes him a great tank.
Also this is my first build so dont downvote unless you have tried it and give me good feedback as in why.

I was going to explain the runes but they are pretty much explain them selves do to the build your using.

I go with a 0/21/9 mastery build for Amumu because it is the best for tanking your getting all the necessities and your getting a boost for your mana and summoner spells.

Use the items for each build depending on whether the other team has more magic damage or more physical damage dealers but if they have around an even amount use this build
but no matter whatis a must for both builds it will make yourslow the enemies down so the rest of your team can finish the job or you can go in for the kill withand finish the job yourself.

I know this build lacks some things but its my first build so i will work on updating it and adding more depending on your comments so please leave me feedback and i hope that this build works as well for you as it does for me.