Constructing an Enchantment Table is something you will do late into the game. It isn't required to defeat the ender dragon, but highly recommended, as it upgrades Swords, Pickaxes, Shovels, Axes, Bows, and Armor.
Anyway, lets get down on what you'll need. You need the following:
The book can be extremely easy, go into an NPC Village and look for a house with some bookshelfs, break these and you get around 3 books per bookshelf.
The Diamonds are obtained via mining, you must go deep into a cave and search for these blocks. They can only be broken with Iron, Gold, and Diamond pickaxes. Although it may be unlikely you will have a diamond pickaxe at the time.
Obsidian is obtained from combining Still Lava + Water. It will become a purple block. This can only be broken with a Diamond Pickaxe, which is gonna be lengthy. It will take around 8-10 seconds to mine one obsidian block.
If you're wondering how to make Obsidian easily, just collect lava in one bucket and water in another, place lava down and quickly place water on the same block, it should turn into Obsidian.
After this has all been accomplished, go into your crafting table and craft this Enchantment Table. Achievement Unlocked!