im asking becuase i have ridicilous magicka cost & chance to cast when i make custom spells... will i be ok with breton apprentice?
Early on custom spells can be pretty expensive. Just go off the preset ones you can buy.
Check out all the Mages' Guilds and Temples; the preset spells in this game are actually pretty fairly powerful, early on. When you're finally able to afford the custom spells, they'll be worth it, of course.
EDIT: a side question - what can i do if my enemy resist most of my spells? Weakness to magicka or i just need higher skill level?
It can be a few factors, but mainly pay attention to what race or type of creature you are fighting. If you're fighting a Nord, use *anything* but cold, as they're very resistant. Likewise with Dark Elves and fire.
If you want to lower resistance, it's good to go with weakness to [element] while using Destruction.
If you're using Absorb Health, Damage Health, Drain Health, Absorb, Damage, Drain effects, Blind, Burden, Disintegrate, Sound, and any other Weakness effects, use Weakness to Magicka.