Wear a full suit of Iron Armour.

Use a saddle to ride a pig, then have the pig get hurt from fall damage while riding it.

Harvest some leather.

Construct a Nether Portal.

Throw diamonds at another player.

Use Shears to obtain wool from a Sheep.

Catch and cook a fish!

Attack and destroy a monster.

Construct one type of each tool (one pickaxe, one spade, one axe and one hoe).

Acquire diamonds with your iron tools.

Smelt an iron ingot.

Construct a better pickaxe.