You have an idea; you've chosen your genre. The next step is to pick your primal fantasy. A primal fantasy is a basic human urge, an archtypal desire that man or woman, from his/ her beginnings on, has needed to fulfill for his/her own happiness. Every book must have a primal fantasy or its characters action have no place to go; it will not hang together, nor will it satisfy the reader.
The primal fantasy that I chose for Jennifer Aiden in Destiny of the Wolf was her desire to become human. To marry the man that she loved and to bare his perfect children. This was her desire for a new life as a human.
To become human, she knew that she had to develop a serum from her biological father's DNA, and to control her step-brother, a vicious killer of humans, whom she feared, one day,would have to destroy him to save the wolf pack, her family and herself.