This is a guide for a general tank Rammus.

Rammus has EXTREMELY low hp for a tank, but he makes up for it with his Defensive Ball Curl and naturally high armour. If played right, you will be near invincible in team fights and no one can escape from you. You will not be able to lose 1v1 fights, not even against Warwick.

I chose magic penetration marks because most of your abilities deal magic damage
Dodge/health/armour seals are all good
Magic resist Glyphs
Hp/movement speed quints

0-21-9

You can't really do anything else for tanks.

Ninja Tabis are good for Rammus if the other team is mainly AD. Get Mercury treads if the opposing team has alot of CC and are magic based. I usually get boots of swiftness because it works very well with powerball.

My starting item is usually a regrowth pendant with a health pot. First trip back, I get the tier 1 boots. Later on, I upgrade the regrowth pendant into a Warmogs armour.

You should try and get the Warmogs armour as early as possible because of the bonus hp and hp regen bonus from killing creeps and champions. The massive health regen is also very useful and with the Warmogs armour, you can lane forever.

Items for Rammus are very situational. Most of the time, you will find yourself against a team of both magic and physical damage dealers. Depending on who you are laning with, you will get different items. If you are laning with someone like Tryndamere or Renekton, you should get Thornmail after Warmogs Armour. It is very cheap and gives 100 armour. The 30% damage reflection lets you punish the opposition for hitting you.

If the other team has someone who stuns alot such as veigar or annie, you should get a Banshee's veil or Quicksilver Sash. I recommend Quicksilver Sash because it is much cheaper, gives decent magic resist and has a useful cleanse active. Force of nature is also a good magic resist item, get this if you need flat magic resist.

If you are dominating, you should get a frozen mallet for the extra 700 hp and the slow effect, which stacks nicely with the slow effect of Powerball.

Your passive ability is very useful. You get 25% of your armour converted into attack damage. Just the other day, I vsed a whole team of DPS carries and the outcome was hilarious. I stacked about 400 armour (550 with Defensive Ball Curl) and taunted Tryndamere who ended up killing himself. He shat bricks after seeing 70% magic damage from thornmail on the death recap. Those squishy AD carries ran at the sight of the Rammus. Moral: Don't pick a team full for AD because there might be a Rammus on the other team.

Put your first point into Powerball as you will need it to get first blood. There is no point in maxing powerball first because it doesn't make you go any faster and only increases damage and slow.

You max Taunt next because it prevents the other team from tower diving your low hp squishy ally. Thousands of times have the enemies tower dived my lane mate on low hp and got taunted and killed by me. If you see a melee champ, taunt them, then run back to your tower with them following close behind then and watch the tower kill him for you.

Defensive Ball Curl is very useful for team fights because of the 150 armour and 150 magic resist it gives. It is the main reason why you are so hard to kill and without this skill, you will not be a tank.

Tremors is a great skill on someone like Rammus. This skill is also useful in team fights and chasing. It is non channeled so you can pop a Tremor and use powerball to chase someone down. If the enemy is in your Tremors for all 8 seconds, they will lose at least half of their hp. It is very easy to keep them in range of your ultimate. First, you use taunt, then after taunt wears off, powerball them, which slows them and has a 0.25 second stun. after that, you should keep following closely behind them, which shouldn't be a problem because of your movement speed runes and boots of movement. By then, your enemy should be on less than 50% of their hp. Taunt them and keep whacking and you will have your kill. Remember to use Defensive Ball Curl because not only does it give you Armour and Magic Resist, it also gives you attack damage because of your passive.