Admiral,
Rammus was the first character I got good with, I played him as a Tank only.
For runes, I gave him all armor runes, ended up with around 15 base armor and another 44 at level 18.
For my trees, i went 21 defense and the rest into utility.
As for his items/spells specifically:
I would start with the 475g health regen, great for melee guys as a first item. Then I would get some boots and some mana regen because i use alot of abilities with him. After that, I would go straight for guardian angel, which I think is the best item in the game for a rammus.
After the GA, you want to get either force of nature if thier heavy hitters are casters or thornmail if they are physical damage.
The aoe 40hp/sec cloak that i forget the name atm is a good buy as well.
For upgraded boots, i like the mobility, so that you can try to get to every battle with those boots and your roll, you should try to be assisting with every kill by about level 10.
For starting spells, I go with the shield first in case we do a 2v2 fight at the start, i'll enter the tall grass first to draw thier fire then pop that shield and take almost no damage as we kill them.
Second spell get your roll, then take 3 levels of taunt for 3,4,5. At level 6 take your ultimate, then taunt again at levels 7 and 9.
In case you haven't caught on, taunt is your most powerful spell, it's a 3 second stun that allows you to pull them anywhere you want, the best thing to do is to grab one guy, preferably a damage dealer with low defenses and run straight at you base, and let your teamates free kill him for 3 seconds as he blindly chases you.
If there are invis chars, make sure you get an oracle early to help your team as you are the tank. Also if you think you can win a fight, you initiate it with roll, shield, ult then taunt.
Also once you get good at dodging between creeps to hit your intended target while rolling at high speeds you will become a deadly rammus.
