The Grace of Wild Things, by Heather Fawcett

Grace's wild imagination and her conviction that she is skilled at magic hasn't exactly endeared the young orphan to any adoption prospects.  So she decides to run away from the orphanage and present herself to the local witch.  At worst, the witch might try to roast her in her oven (which she in fact does), but it would be better than having her wild imagination suppressed.  

Grace manages to convince the witch to give her a trial -- to cast the 100 (and a half) spells that are in the witch's grimoire.  If Grace can do this before next spring, the witch will take her on as her apprentice.  If not, then the witch will take away any talent Grace has for magic.  It's a challenge that seems insurmountable, but Grace is as determined as she is melodramatic and with the help of a few friends, she works through the task.

Some elements of the story will feel quite familiar.  Inspired by the classic Anne of Green Gables, many of Grace's adventures (from smashing the school bully with her slate to accidentally intoxicating her best friend) are taken from Anne Shirley's mishaps.  The setting on Prince Edward Island is also the same.  But Grace, while prone to Anne's loquaciousness and inspired imagination, is an aspiring witch and this story is magic fantasy with faeries, ghosts, and spirits.  That Fawcett manages to also work in the themes of imagination and found homes is a marker of the cleverness of this novel and a testament to the universality of the inspiration.