You can open your spellbook by pressing (by default) "O". Here you can find all spells that are currently available to you. Also listed will be what reagents are required to cast each spell. When you "cast" a spell you actually pre-load it so the spell is in a standby phase; after this you can aim at your target and press the button again to cast your spell. All spells have a chance to fizzle (fail). If you equip too much armour the chance of your spell failing becomes much higher.
Intelligence and Psyche are the primary attributes that affect Magic.