As you progress into the middle stages of Minecraft (see Tutorials/Starting the Game for help at the start) you will be reaching the peak of your industry, creating many of your weapons, tools and utilities for later game.
Metal of Industry
When you have Leather (or Chain, but that's unlikely) Armor, venture into the shallower caves. Mobs may appear, but you can fight them off with your stone sword while keeping your stone pick ready for mining Iron Ore.
When you find it (it appears as a stone block with tan flecks in it), mine it with your stone pickaxe. Keep an eye out behind you for mobs; Torches can help for this purpose. When you get enough (you need 29 ingots to craft the full set of armor plus the pickaxe and sword; might as well round it off to 32 to avoid wasting coal), venture back home, preferably with coal or charcoal to smelt the iron ore in a furnace. Once you've done that, your priorities should be:
- Iron pickaxe
- Iron sword
- Iron chestplate
- Iron leggings
- Iron boots
- Iron helmet
- Other tools (hoe first, unless you're a nomad)
Iron is probably the most useful tool material in the game, so you'd better use it well.
You may want to expand your base a bit here with all the cobblestone you've collected down in the caves. See Tutorials/Building a Base for this.
Take to the caves!
Now you're on the ultimate quest: Searching for diamonds deep in the caves. This will be difficult. What you need:
- Lots and lots and lots and lots of Torches
- Iron armor at worst
- Iron sword and pickaxe
- Stone shovels (optional)
You'll find them down around level 12 (look out for lava!).