Build depends what kind of team you're fighting against. I pref taking 1-3 Health Potions at start so you don't have to go back get HP and miss lots of XP.

Easy Team

Well, you can go what ever build you like. AD or AP or tank or DPS tank... Atma's Impaler is good extra to your build if you go for tank / dps tank build.

Magic Team

If other team has a lot champions who deal magic damage you go for magic resist build, but if they're not good you can get around 50% magic damage reduction and get something to help with getting them down. But if they're good you go maximum magic resist and try keep them hitting you (taunting) and not your teammates.

Physical DMG Team

Almost same as Magic Team but you get maximum armor if they're good. Thornmail is good; you get armor and when enemy does damage he get's damage also! Frozen Heart is also good extra for your whole team if theres some attack speed monster such as Jax/Master Yi.

Hybrid Team (Physical DMG + Magic DMG)

You should go 50-50 with armor and magic resist, but if magic dmg is greatly high than Physical, more magic resist and vice versa.

Remember basics

Keep taunting enemies so they attack on you and not your teammates and enemies who comes too close to your turret ;) Best way to enter into team fight is either to use gang surprise from bush so you can taunt couple enemies and then they have hard time to target your teammates OR use ultimate to teleport right into business and taunt them all! That way you can maybe save someone. Good tactic is also to recall back to base and make sure that enimies see that. And when you're full HP and enemies thinks your buddy is alone and buddy attack, then you teleport onto him giving him bubble and you can taunt enemy/enemies and get an easy kill.