Helps that he isn't that expensive, so he's a fairly easy buy.
Someone prior to me said CDR, but mana is a big necessity, and he can faily easily pick up a Frozen Heart or Iceborn Guantlet and get decent synergy with it (i recommend Iceborn for the extra AoE damage and slow, just gotta get the spell-auto weaving).
Because of his need for mana, don't spam Q, at least not until after you have one of the two above or Glacial Shroud (component of both). Rank 1 Q uses about 1/4-1/5 of his mana pool, and can room out of mana quickly when spamming it.
Personally, I focus on his W, as it increases his armour by a percentage, increases his waveclear due to AoE autos when activated, and it also increases his E's damage due to the increased armour and E's armour ratio. Max W you can end up over 200 armour with one armour item...depending on which one it is. From there you can either build more or itemize differently depending on what you're up against (Warmog's is also nice for him).
When in lane, play around his Passive shield as best as you can, since it's a few extra autos you can soak up when farming. The shield is equal to 10% of Malphite's max health.
His ult is the key. Very fast AoE knockup. Too strong in my personal opinion and why i want him to get more work done, but it is what it is for now. As said elsewhere, it is a great engage, especially for wombo combos (yasuo loves malphite's ultimate).
Once you're in the fight, if you went with Iceborn Gauntlet, weave your autos between abilities as best as you can to get the Sheen procs for slightly more AoE damage and an AoE slow with a fairly wide radius due to Malphite's bonus armour rating.