Magic in the Warhammer universe is rather different than what you may be used to in other settings. Instead of a set use limit, or a replenishing mana system of sorts, all magical power in the Warhammer world originates from the Realm of Chaos, and seeps into the material world via the highly unpredictable 'Winds of Magic'.
Magic in the Warhammer world is also normally assigned into set 'lores' of study and thought, so for example a pyromancer is someone that has devoted themselves into the Magical school of fire and so forth. It is noteworthy however, that magic here is highly unpredictable in general, and miscasting, or worse backfiring, can readily occur with ease given the very nature of the Winds of Magic. Nonetheless, magic is a highly powerful and commanding force on the battlefield. A user of magic, is most commonly referred to as a 'mage' or a 'wizard' in this setting, however racial variations, such as Chaos Sorcerers, High Elven Loremasters, and even Beastman Bray-Shamans all exist.
http://whfb.lexicanum.com/wiki/MagicGiven that normal magic on the table-top version is assigned into rigid spells that leave little to the imagination, I would like to leave spells up to your imaginations. Of course, logic is prime here as a mere apprentice would not be going around immolating whole armies or the like. I would also like to leave the random chances of failure up to you, however I can swiftly provide them should it be wished for.[/spoiler]
Notes About The World Itself
[spoiler]The technological level of the general races in the Warhammer universe, is best compared to Early Modern, or early Renaissance in nature. Many factions, especially Dwarves, Skaven, and the Empire of Sigmar, all have ready access to (relatively) stable and cheap firearms that work much in the same manner as normal matchlock, Wheelock, or flintlock muskets. However, all factions still maintain strong cores of melee infantry and the like, and most factions actually have no standard or formal access to these firearms, though certain individuals among them may do...