Playing Rammus can be fun when tanking. Have you ever thought that he could be the one getting the kills? Going AP with Rammus is a great idea because his Powerball uses all of his ability power and his Tremors takes enough ability power to destroy the whole team in a fight.

I use these runes because they will give you enough power and health to get first blood. After you get a kill or two you will be able to get your build.

Choosing the offense and defense trees for your masteries work well because your going to want to be tanky and powerful at the same time.

Its always a good idea to start off with a dorans ring when going AP Rammus. His powerball takes quite a bit of mana and it can get annoying when you can't powerball that Teemo your after.
Boots of Swiftness or Sorceror shoes work well with Rammus. So choose wisely depending on the team your going against.
Rod of Ages is my favorite choice for the first big item because it gives you enough mana and health to last you awhile in team fights.
Rylais will give you more health and help slow enemies with tremors. Its a useful item but not necessarily needed early on.
I get Rabadon's Deathcap after Rod of Ages because you need more Ability Power. By the time you get this item the other team may or may not become tanky.
Void staff will work well against those tanks that never die.
Zhonyas Hourglass can replace the dorans ring so you can roll into a fight, Tremors, then activate hourglass and by then they should be dead.

Flash- Works very well when ganking. Powerball Flash FTW.
Ignite- Finish off those noobs when low health obviously.
Exhaust- Good for using Tremors.

Add me- pandajackjack