Go to the Object Editor (OE) and make custom spells using only Object Editor features.
As soon as you understand what happens on those things, you move on to the Trigger Editor.
In the Trigger Editor, you find out what "Game - Display <string> to <player group>" does.
That is your only friend in the Trigger Editor.
Even the best programmers cannot do without it.
Then find out what all the events regarding spells do:
- Start casting
- Start channeling
- Start the effect
- Finishes casting
- Cancels casting
(you will use "Start the effect" most)
Then you go and try to deal the damage or heal a unit using triggers.
Then you go and try to make those heals/damages increase by stats such as STR, INT, AGI, Health, Mana, Level, etc.
Then you go and try to make special effects.
Then you go and try to make Area of Effect spells work properly.
After that, you can try to download systems that will increase the possibility to make better spells.
I guess that will keep you busy for at least 6 months.