Malphite is in my opinion one of the best tanks in the game. He has a natural shield which actually gives him 110% base health. His ground slam hits extremely hard and his slow goves him a speed buff. his ult knocks everyone in the air ; this means a total control over the enemy team in team fights.

You are a tank. Your goal is to initiate the fight to get focused, allowing your team to hit the opposite players. Be careful though if you are not speaking to them (via skype, teamspeak, mumble, whatever) and if you see them hesitating, as they might not follow and you might end up as a free kill. You should either initiate with your ult if your team in not far behind you or if you are late at entering the team fight, or just start by slowing one of the enemies down and engaging him if your team is far behind. Do not use your ult in these circumstances as the bump will be useless in one of its two main points : slows their retreat and prevents the enemies to hit your team. In any case, when you are in the middle of the fight, use your Brutal strikes to get extra armor (and damage) and then Ground Slam as it will deal a significant amount of damage to all enemies around. Exhaust their carry to minimize their damage and you should win most equal team fights.
If running use your ult as a last resort because of its high cooldown, try using your seismic shard first because of the slow and your boost if they have no cc left.