So you've found the stronghold and you're just about to enter, but WAIT! Before you hop in that dark dangerous portal of death, think. Do you have all you need? Here is my list of items neccesary to defeating the Ender Dragon! If not prepared, i suggest making some sort of landmark to remind you where the End Portal is.
Wooden crap won't save you now! Make sure your equipped with a Diamond Sword, and make sure it has a good Enchant, such as the Infinity one. Bows are a must in The End, for they deal quite a lot of damage, and the Dragon can, well, fly, so you can get in a couple hits whilest he's flying. Plus, Long Range crystal attacks are great, due to the explosion they make. Don't forget to have a least 1 stack of Arrows, and make every single one count! If an enderman attacks, don't use arrows. Save them for the Dragon.
Diamond Gear is also neccesary to defeating the Dragon! Optional, but if you want to keep Endermen from attacking whilst on the ground, equip a Pumpkin so they won't bother you, and you'll get around quickly.
This is going to take a long time! Make sure you're all stocked up on Food, because your hunger bar might go down like a rocket while fighting! Also, make sure you have Apples. They can heal you during the fight.
There are giant crystals healing the Dragon, but how on earth are you going to get up there? Ladders. Lots and lots of ladders. Use the ladders to climb up the Obsidian towers, and stand back, because that crystals gonna blow up like firecrackers!
Now you are all prepared! Good luck out there, and be sure to always keep a good supply of arrows!