https://kingdomhearts.fandom.com/wiki/Deck_Command
Here's a tip - dodge roll a lot. Aqua and Ven's dodge rolls have a crazy amount of invincibility frames (Terra's doesn't). In general, make sure to meld commands together to get abilities, and of course do some grinding. Playing the command board is a great way to level up commands quickly, and there are also ways to get broken commands early in the game through melding. You can go online (or onto this wiki) to get the recipes.
Here are some cool commands and how to make them:
Triple Firaga or Blizzaga (Aqua only) - Meld two Firagas or Blizzagas together (melding a tier three magic command with a tier two one might work)
Magic Hour (Aqua and Ventus only) - https://kingdomhearts.fandom.com/wiki/Time_Splicer
Zantetsuken (Terra only) - https://kingdomhearts.fandom.com/wiki/Zantetsuken
Thunder (and other elements) Surge - https://kingdomhearts.fandom.com/wiki/Thunder_Surge
Ars Arcanum (Ventus only) and Ars Solum (Terra only) - https://kingdomhearts.fandom.com/wiki/Ars_Solum
https://kingdomhearts.fandom.com/wiki/Ars_Arcanum
Seeker Mine or any other mine attack (a great command - you can lure the enemy into it) - https://kingdomhearts.fandom.com/wiki/Seeker_Mine