Yeah so this is my first build.
Rammus is an all around armor character because his passive gives him damage for 25% of his armor. What i do for rammus is get armor at the beginning and finish off with health.
Pros:
Massive armor so you won't take much damage from melee attackers
His armor gives him a lot of damage
His ultimate can do a lot of aoe damage with a low CD
He has a spiked shell and he spins!

Cons:
Not too much spell resistance
Casters can damage him pretty easily unless you are in defensive stance.
Pretty much All around bad for casters if u dont build spell resistance

Only fight alone if you have a lot of the items listed, and possibly a level or 2 higher.
When fighting, you should use your abilities in this order, Q W E R. lol thats easy
You can use your Q spell to get away because it periodically speeds you up. up to a maximum of around 780 i believe.

You need to get armor runes for rammus because the more armor he has the less damage he takes from physical hits, and his passive ability is that 25% of his armor goes to damage. also his W spell increases with armor.