I'm going mad...anduboy, please, can tell me step by step how to precast a spell and then using it by pressing 1, 2 or 3?
I'm at the beginning, i only have the fire spell.
Thank you very much...

I assume you purchased the game here. The manual can be found under My Acount, My Games, Downloadable content for this game.
I know the answer to your question, but I think you need to find it yourself.
Hint: Look at page 56 of your manual.

Of Course I buyed the game here (and it's the seven one here... and many more will come, GOG really kick ass, i was very bored of Steam and his "half of an hour to start a game").
I finally found the solution myself, but thanks for the hint.

hold down ctrl and shift while you cast the spell. after its been prepared it can be cast by aiming at your target and pressing ether 1,2 or 3. for those who like me, just want an answer to a fuckin question.