Getting down to details-when a character is ready, willing and is able to use magic. You say what is next? Spellcasting takes some preparation and some thought. If you thnk tossing fireballs left and fight at a drop of a hat sounds cool, I hate to say it doesn't work that way. Casting a spell the character must have a purpose, do a bit of research, make any ritual preparation and gather any ingredients and finally do the deed for good or bad.
Purpose
Before mumbling some wacky words and making hand gestures, the character must know what he/she wants to do.You feel this maybe easy but it isn't, because if your intentions are too vague, the powers invoked by the spell may be "interpret" them as they wish-which is a rarely good thing.
Example: Saying "I want the to make sure the ghost doesn't bother me" sucks as a "Mission statement". Saying such a spell might do almost anything like kill the caster, which at that time the ghost will trouble the caster no more. "I want the ghost banished from this mortal plane never to return" is alot better and clear on your intentions.
So what can a mage wish for? In Theory anything, but in practice not so much. Magic can effect living and non living things. it can heal, destroy, manipulate minds, matter and energy (but there are limits). Now these limits can be as followed like making a objectappear out of thin air is next to impossible.
Example: Magic can transform or destory things that already exist, rather then creating something out of nothing. Doing permanent effects are more difficult then doing a temporary spell. (turning paper into cash can be done, but may change back after a short time) More ambitious the purpose, the harder it is to find a spell that can accomplish it.(The best spells are simple and straightforward)
Now spells that are meant to squash enemies like a bug or help overwhelm all challenges in a scene are not going to be available for the most part (when they are, they exact a high cost from those who use them). Magic can't solve all or most problems that would be to easy and boring for a rping game.
Power Level
The power level is set by the director for when a character casts a spell. (Each magical invocation has one) The power level is determined by the overall strength of the spell. A spell for hiccups will be much less difficult then one to raise a barrier that is impenetrable and around a building. A spell that has a higher power level is harder to cast property and the damaging the consequences of failer.
Figuring out where to set the power levels at is a judgement call for the Director, then there are always power level 1. No matter what you call them be it cantrips, orisons or just plan old level zero spells (which is just plain silly). The most powerfull spell a cast member can cast or try to cast is a Power level 10 (this is a warning but your Director will warn you on trying a spell this high. Because even with liberal use of drama points the side effects are very serious)
Effect
The easiest way to start focuses on the effect desired. That creates a base Power Level.
Power Level 1:
A spell that doesn't significantly alter or damage a subject (living or non living). These kinds of spells annoy or amuse, but doesn't heal or hurt people. example: like forming a small globe of light to see in a darkroom.
Power Level 2:
A spell this strong to injure one person, break several small things or produce a impressive light show. Example: to inflicting damage is figured on the caster's willpower for every success lvl rolled. (Shattering all glass or other fragile objects in an area the size of a room or a small house.)
Power Level 3:
The spell this lvl can be acharm to alter a person's emotions and senses, inflict severe injuries and reshape matter (but not trasform). A spellcaster can summon beings from another dimensions at this lvl. Example: Paralyzing a victim (reduce dex by one per success); Inflicting damage in the amount of two times willpower per success level; changing a person's feelings (a love spell); etc...
Power Level 4:
Spells of this lvl can transform a living being, reshape his very essence and soul. Example: Turning a person into a animal or into a demonic creature; Summoning a powerfull demon; inflicting damage at a rate of three times willpower points per success level; etc...
Power Level 5:
Spells at this lvl can do incredible things, such as restore a vampire's soul, throw lighting bolts, create walls of impenetrable force, or summon a world eating demon for fun. Example: Doing damage to a person by willpower times 5 points of damage per success lvl; bringing back some one from the dead; etc...