Start With the Right Tools
Before going mining make sure you have iron pickaxe at least. Stone is good for coal but not for gold or redstone. Bring torches food and a sword because caves are not safe.
Go for Strip Mining
Strip mining is best way to find diamonds and other ores. Dig down to level Y -53 and make long tunnels. Leave 2 blocks between tunnels so you dont miss anything. It takes time but gives best result.
Branch Mining Is Also Good
Make a main hallway and then dig side branches every few blocks. This way you cover more area and waste less time. Use this with strip mining for better chances.
Light Up Your Path
Always place torches while mining. It helps you see and also stop mobs from spawning. If you see dark place always light it up first before going in.
Bring Water Bucket
A water bucket saves life in many ways. You can turn lava into obsidian or stop yourself if you fall into lava. It also help in climbing down caves safely.
Dont Dig Straight Down
This is the oldest rule in Minecraft. If you dig down you can fall into lava or cave with mobs. Always dig in staircase shape or two blocks side by side.
Use Fortune Enchant
If you get a pickaxe with Fortune enchant it will give more drops like diamonds or coal. Try to save diamond ores until you have Fortune tool.
