Minecraft wiki entry for The_second_day, associated text: A shield. This requires several wood planks and a single iron ingot. Shields were introduced fairly recently, but have seriously changed the balance of power against mobs: You can use them to block skeleton's arrows and hold off most other mobs' attacks, including creeper explosions.
An iron pickaxe, to mine more advanced ores (see below) as you encounter them, which are key to proceed through the game.
An iron sword, to attack monsters much more effectively than a stone or wooden sword.
A bucket. This lets you pick up water source blocks and place them elsewhere. This is a key ability for making a farm, and it lets you generally rearrange water in the landscape. You can remove or place waterfalls, create your own ponds and rivers, (useful for fishing), defend your base against creepers, and more complex purposes. You can also use it on lava and milking cows.
Iron armor, which gives you much more protection against monster attacks and most other damage. The full set requires 24 ingots. If you don't have that much yet, make the various pieces of your armor set as you get more, starting with the chestplate.
At this point, you have used 33 ingots. If you have iron left, go on to make shears and a second bucket (to hold a second water source, milk a cow, or get a bit of lava out of your way).
Set aside half a stack or so (that is, 30-odd ingots) for when you need to replace some or all of your equipment.
As you continue to gather iron, you can start switching over to iron tools for everyday work: An iron pickaxe, shovel or axe works faster and last longer than the stone tools. Extra buckets come in handy too.
Once you have redstone, make a couple or few compasses (the extras are for maps).
Eventually you can make an anvil for 31 iron ingots.