Writing is a discipline. Much like training for a marathon or learning to dance or learning the guitar. There is no substitute for preparation or continual effort.
Don't worry about your story idea being "not new". In fact most story ideas are as old as time itself. It's just how we tell it, the setting. Boy meets girl stories. Heist movies. Wedding stories. Shakespeare's stories are always rehashed every few years or so.
Yes, audiences always want something fresh. Just like people want to taste something new. But in the end there are only the basic food groups. It's how we cook it.