There are 3 hotkeys; Q, E, and R. They correspond with your weapon, main charm, and accessory charm, respectively. So if you have an intelligence dagger in your weapon slot and an attack charm in your main slot and a heal charm in your accessory slot you just need to press R to switch to your heal charm and then simply click on a monster while in combat to heal yourself. To heal other people in catacombs just make sure that you have your heal charm selected and click on their health bar while in combat.
If you have your heal charm in your main slot then just press E to select it. If you're interested in reading a magician welling guide I wrote you should read
this.