Sion is a really tanky champion, and he can get over 5k health easily, but that doesn't mean he's unstoppable. Here's a guide that will [probably] help you play Sion on top.

Grasp of the Undying - Healing, increasing max health, bonus damage - Everything Sion needs.
Demolish - This rune can make pushing a lot faster.
Iron Skin - Pretty good for Sion, especially when your enemy deals physical damage.
Overgrowth - More health increasing! Makes you more and more tanky over shorter amounts of time when combined with GotU.
Cheap Shot - You have two slows and two knockups/stuns. Really useful.
Zombie Ward - Makes you aware of ganks for longer.

Ninja Tabi - Armor+Further damage reduction. Useful against both AD and AP champs.
Warmog's Armor - Goes well with his ulti on 3rd level. When the teamfight goes rough, back off for health, and ult the enemy team again.
Spirit Visage - More regeneration + magic resist grants increased survivability during teamfights.
Frozen Mallet - Slows enemies and lets your allies take care of them. Also, 700 health!
Sterak's Gage - Lots of bonus damage, because Sion's base AD is really huge.
Randuin's Omen - Reduces incoming damage, grants armor and health. Late game item.
Dead Man's Plate - Even more damage and health. Cool name :D.
Sunfire Cape - It's a great choice, but only for late game, because most of the turrets should be gone by now.
Gargoyle Stoneplate - The god of all items during teamfights. But reduces your damage by a lot.

Sion is a really heavy champion, and he deals lots of damage to those who stand in his way. However, he's weak against mobile champions (Teemo, Master Yi, Yasuo) or [in midgame] those who create obstacles(Jarvan, Azir, Anivia, Ornn)