Hold a button or 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 potions or skills with a long cooldown or high mana requirement, just in case.
If you use a melee or range attack/skill, your character will automatically switch to the appropriate weapon, if needed.
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.
Welcome to the forum.
