How to find Lava, steel, Diamonds on Minecraft Pocket edition on Iphone?

"How do you find Diamonds?"

For Diamonds dig down to bedrock, then start using mining shaft method to explore.

How do you find lava?"
As for lava, just explore, as lava only spawns on the surface until patch 0.9 comes in.

Both substances are very, very rare in Minecraft PE. Far rarer than they are in the PC version at least.

"How do you find Steel?"
Steel does not exist in Minecraft PE, it doesn't exist in PC Minecraft either. The item 'Flint and Steel' despite the name is made with Iron