Walkthrough:Kingdom Hearts Re:coded/Darkheart3/Destiny Islands

Awakening
You will begin in a place called Station of Awakening. It basically acts as a tutorial for the game. Walk up to the shiny object. You will be presented with 3 weapons. Depending on what weapon you choose, and what weapon you give up, your stats will be somewhat different. Make your choices, then follow the guy in the black coat up the stairs. On this next platform you will learn to fight. Defeat the swarms of heartless, then examine the door.

Destiny Islands
You will end up at the Destiny Islands. Walk up to Selphie, and after hearing what she has to say, make your way down the beach to Wakka. It seems he's discovered a new type of block. Advance through the Metal Blox road, and hit the block 3 times. Yay! You got a prize. Go and install it in the Stat Matrix. Then jump down to Wakka and tell him of your findings. Then, head up the ramp and use the Metal Block to jump onto the roof of the shack. You will see that guy in the black coat again. Walk up and examine the door he disappeared into, then enter the portal. Now you will learn about System Sector. Jump down into the middle, and defeat the 2 slightly distorted heartless, then use the link to the next floor. There are a lot of heartless on this next floor, so unleash your rage and destroy anyone who dares get in your way >_<. Afterwards, use the terminal to exit the sector. Grab some prizes on your way out, then walk across the repaired bridge to Tidus.

Entering the Keyhole
Grab the prize block to your right, then head over to that little alcove by the waterfall, where you'll have your first real fight.