This is my first build, so if you do rate please leave helpful comments. Thanx

Getting to Rammus, he's a odd tank in that he's squishy before he reaches level 6-7.

Summoner's Spells
Using ignite with Rammus will help with getting in the extra damage needed early game for assisting 1st bloods and in later game killing squishy heroes.

I picked Heal for Rammus because with him being squishy also opens him up to being an early game target, this helps with stay ability within a lane as well as keeping your teammates alive mid to late game in team fights.

On with his abilities:
Passive Spiked Shell - 25% of your armor becomes additional attack damage. Meaning this ability interacts quite nicely with defensive ball curl. This ability is what let's Rammus deal so much amazing damage while being relatively untouchable in late game.

Powerball - Useful slow, knock back, chase, and escape ability wrapped into a comically correct rolling armadillo. It deals decent damage and slows early game but doesn't help enough later game, so don't bother putting more than one point in here. This ability will help save team members on the run by slowing and knocking back their pursuers. Use this ability mostly for utility, positioning, and initiating ganks and team fights.

Defensive Ball Curl - Believe it or not this ability will help you deal most of your damage and increases your survivability amazingly. Especially when heroes such as Master Yi or Warwick try to solo you. The damage it returns per hit will stack up quickly and when they realize they're taking too much damage, simply ultimate and taunt to finish the job. This ability is also great for enduring early casters such as Nunu and Rhys with the added magic resist. Remember, this ability interacting with Rammus's passive let's him deal extra damage.
Please note that this ability no longer uses ability power, it merely adds 10% of your armor to a base amount.

Puncturing Taunt - Rammus's most overpowered ability. This will help when people get too gutsy with towers you are defending. When a hero gets too close to a tower, simply rush in from the bushes with Powerball, taunt them, then laugh as they get their behind handed to them by the tower. Also use this ability for saving your squishy members of your team while being chased or during team fights for making sure that the opposing team's damage unit focuses on you.

(Disclaimer: Make sure you have enough health to handle the damage before you try to save someone. It's noble to sacrifice for the team but unfortunately doesn't get you a good reputation.)

Finally Rammus's Ultimate: Tremor - The reason Rammus is invaluable in team fights. To everyone in an area around Rammus, this ability deals 520/ 1040 / 1560 (split between the multiple targets) as magic damage over 8 secs. This ability coupled with his taunt will help demolish in team battles. Simply activate once properly positioned near the majority of the enemy heroes and watch as your enemies get picked off by your teammates.
Make sure to read this ability carefully, this ability causes "damage to all nearby units and structures". Wait what? "units and structures" .... THIS ABILITY CAN TAKE OUT TOWERS!?!? HECK YEAH!

On to game play:
Getting Doran's Shield and a health potion will help with being able to stay after being harassed by heroes with ranged attacks or abilities.

Early game grab powerball, for running when necessary and for having the option of burst damage and slow to help lane partners kill people. Also note that this provides a very large speed boost, meaning whenever you are at the Summoner's circle make sure to powerball out for extra speed at no cost.

Concerning which boots to buy: Depending on what kind of team your facing get Ninja Tabi for a lot of AD champs or get Mercury's Treads for high CC opponents.

If you are solo for some odd reason, tower hug and scare your enemies with taunt into a tower.
Once you start leveling try to use towers and your Defensive ball curl to your best advantage.

Getting your ultimate will help you push towers nicely. Also, if you have the opening, use your defensive ball curl to rush the tower with extra damage. These two abilities help rammus become your team's secondary building killer.

Mid Game (Lvls 7-12, or 10-20 minutes)

By now you should've gotten your Ninja tabi and Sunfire cape and will be starting to construct your next item. Your 3rd item will vary depending upon your opponents. Alot of CC? Get Banshees veil a little early. A ton of fast attackers? Eat them alive with Thornmail. Use Force of Nature for casters that don't have targeting spells or for increased survivability and regen.

Rammus with Sunfire cape can be a very effective jungler at this point. So if you need extra cash or that little extra experience for the next level. Don't feel shy to powerball into some neutral creeps then defensive ball curl for a quick boost.

By this point in the game you should be initiating ganks on a regular basis and your opponents will be complaining about how broken your taunt is. Aim for squishy people because your ultimate is not yet maxed and you can't take on most other health heavy champions without some assistance.

A nifty tip when facing squishy casters, use your taunt to prevent them from disabling you or your team's carry. Remember your taunt prevents them from casting for a full 3 secs, use them wisely.
Tower destruction will be one of your functions of your ultimate. You can't tank the tower yet.... so don't try!!

Late game - With your build almost complete the only thing you truly need to worry about is if a team of mean casters constantly disable you and cast their heavily damaging nukes.

Once again you are at a decision on what final items to buy.
Randuin's Omen is a nice neutral item that helps slow down the opposing team. I prefer using this in the middle of a team fight to catch fleeing enemies and to render most AD champs useless. IF YOU BUY THIS ITEM MAKE SURE TO USE ITS ACTIVE ABILITY!!!!

Frozen Heart is incredibly useful against all melee champs in that it prevents them from dealing their full potential of damage. The cooldown reduction isn't bad either.

Guardian's angel is here primarily if you noticed you're getting targeted a lot. It's balanced in armor and magic resistance and will help you flee if you are about to die.

The Crystal Scepter is kind of a joke item. If you are wrecking their team then you can get this item to add damage to your powerball and your Tremors. As well as adding hilarious slow, making sure your opponents aren't going anywhere.

Good luck, hopefully you'll have as much fun rolling people down as I do!