After the cutscene, you gain control of Sora, our main character. You're asked to pick between a sword, shield, and a staff. After choosing one, you have to sacrifice another one. Though it's the first choice in the game, it's crucial because it determines your starting statistics and which abilities will be learned first during leveling up. Here are the stats you'll recieve with each combination.
Choose: Sacrifice: Strength Defence AP MP Sword Shield 6 1 3 2 Sword Staff 6 2 1 2 Shield Sword 4 4 3 2 Shield Staff 5 4 1 2 Staff Sword 3 3 3 3 Staff Shield 5 1 3 3
If you want advice on what to choose regarding abilities, that's completely up to you. No matter what, you'll always have the same abilities by the time you reach level 100. As for stats, the choice should be between choosing the shield and staff. If you choose the shield, you will have 2 more item slots by level 100, whereas with the staff, you will have 1 extra MP. (The sword gives you more strength, but that's unimportant as you can easily get and Strength boost item and get maximum strength either way). The choice is up to you.
Once you've made you decision, you'll be transported to another platform. It's time for your first real battle.