Hi fiffa46!
That's not a stupid question at all - and you're not the only player who has had difficulty figuring out how to cast heal or use the healing potions.
For the potions - you have to use the USE/GIVE screen on the lefthand frame. Choose USE, choose the potion - and then control click upon the character you want to heal.
It has been so long since I played, I may have this wrong - I think to cast heal you have to control click on the character to be healed as well.
Check back in a while - if I'm off, someone will correct me shortly!
Wyv