Read other great stories. They can inspire you and make you eager to create something just as good or even better. Also, share your idea with friends or a writing group. Their enthusiasm and feedback can fuel your motivation.
Read other great novels or attend writing workshops. It can reignite your enthusiasm and give you new ideas and techniques to get motivated and keep writing.
One way to get motivated is to read some great short stories to inspire you. Another is to envision how good it'll feel when you finish. Just start writing, even if it's not perfect at first.
Read inspiring quotes about writing. There are many great ones out there that can boost your motivation. For instance, 'The scariest moment is always just before you start.' by Stephen King. This can make you feel that it's normal to be a bit scared and still start writing.
Read lots of great stories. Seeing the creativity and impact of others' works can inspire you to put pen to paper. Also, set achievable goals for yourself and celebrate small victories along the way to keep you going.