100%ing Kingdom Hearts 1

The First in the Story

I HIGHLY recommend KHGuides for all of KH1. I was able to complete all achievements (besides my proud run right now) using solely that guide. If you are going to 100% KH1, there are a couple of things to keep in mind.

MOST achievements aren't missable.
You will need to play the game 2-3 times.
One playthrough will be your "normal" playthrough where you collect all Jiminy's Journal entries, synthesize everything, etc.
The other playthrough will be a special beginner run where you get the Speedster, Undefeated, and Unchanging Armor achievements all at once. It's not hard, don't worry.
Difficulty achievements stack (outside of PS3). You only have to play through proud mode once, but 100%ing everything on proud mode might make you frustrated. I chose to go back and do a final proud mode playthrough later.
So I would recommend playing the game on Normal and focusing on ENJOYING it. Then go back, clean up. Do your special beginner run. Then take a break and do the proud mode run eventually.