Hold a button / pad direction down for a couple seconds and a menu will pop up allowing you to assign skills, potions, etc to them. The actions assigned to the A and B buttons can sometimes get triggered when finishing or exiting out of conversations, etc, so it would be better not to assign skills with a long cooldown or high mana requirements, just in case.
FWIW, playing a warrior I assigned X melee attack, A range attack, Y Rush Attack, B whirlwind, left Dragon Stone, right Dragon Morph stone (both after getting the battle tower), up Heal (level 1 comes in handy) and down summon creature (after getting it).
For potions, I generally just paused (or had the low hit point auto-pause kick in) and hit a shoulder button to bring up a similar menu as with assigning button actions. That is also handy for skills you only use occasionally.
To learn new skill or increase the level of ones you have, in the skill list hit A to show the skill description, and A again (one or more times) to apply skill points; when you hit B to exit you are given the choice to apply the points (A) or cancel (B).
