My first guide please be mean only way to learn :)

some tanks play passive I like to be kind of passive early game if he is not I punish him and make him be passive. Lucky for you Tahm kenches build set is just perfect for this you have stuns slows and even able to carry the enemy or ally away from an area to save or do more damage. I have also found a good poking with him is to eat a minion and throw it at the enemy it has quite a bit of range.
With my starting items I like to go for items that will get me too what I want as fast as possible in this case I want that ruby crystal because no matter what happens if I need to go Into spectrals cowl or bamis cinder. If you going against someone really ad heavy i would say to start with a cloth and five health pots then go chain mail then get your catalyst unless your still having a hard time then i would finish of sunfires cape but it will rarely happen. If you happen to be going against an ap heavy champ like vladamire then i would say to go a dorans sheild health pot and go spectrals cowl then into catalyst because most ap champs should not have an ap spike yet
ROA's i cannot fathom why you would not build this item it gives him everything he could ever dream all of his abilities scale with ap gives health and mana everyone needs boots its just a necessity. I find sunfires to be the best tank item in the game personally it gives defense and damage over time cannot ask for a better item for any tank.
In the rare circumstance they have no ad damage do not go for it though instead go for spirit visage.
ZZ'Rots is too good to really pass it up I cannot see a reason why to not build it the armor and health regain are just too good plus it helps push lane too and speed near turrets is useful Banner of command gives ability power which he scales off and some mr wich is really good plus also gives some good pushing power with empowering minions randuins is a good tanky item just really it on that good for all tanky teams. GA well i probably would never go that but it has good mr and Def so it has some good tanking abilities to it. frozen heart is really good against ad champs and I would probably take that of randuins if the opposing team has a lot of attack speed I would probably go frozen heart with thorn mail because the armor and dealing damage to themselves is very useful.

Abilities what I put up there will be the most common build path but I can have some variances depending on who I am fighting. W is so useful I mean you eat someone and spit them out and it can be used as good poke if you eat minion and can be used for crowd control so I want to max that first and mostly use my q as a stun or slow if I have to cause then I can catch up and eat them so i can move them back into our team or later on in the game use it to get rid of there carries for a short time. If I am going against a heavy damage dealer like Darius or Riven is would be more beneficial to max e first because the sustain will be so much more useful. If going against Nasus eat him before he farms he loses on farm win win right :P and ultimate is always useful to level up so ya do that.

Tahm is a tank and i like the 0 21 9 with him the reason I like having my 9 being in utility is because the mana regen potion duration and Speed is very helpful laning phase and will help you get through it much easier the tank portion is because it Allows him to be as tanky as possible because I believe he should be a tank and be able to go into the enemy team and subtract a heavy damage dealer to make a team fight 4 v 5. so ya not much really needs to be explained here

Honestly this is basically what I run on almost all of my champs i cant really see the scaling to be useful so i never use it the 30 some odd magic you would end up in the end is not really useful so I just do not care about it. I personally think runes only make a difference in early game once the game gets into late game they kind of make no difference to me so i like to run the early defense with mr and either magic pen or armor penmarks with damage or ap quints depending on who I am playing and Tahm scales from ap so I run magic pen with ap

I am so dissapointed they did not quote chef Ramsey with IT'S RAW!