In Media Res

To be completely effective, begin your story at the time of change that threatens your main character’s self concept or that changes their world in some way. This is usually in the "middle of affairs." This technique allows the story to start off at an exciting, pivotal, and engaging time in the character's life.

If your story requires back story, do not pad the story with information from the past, or scenes leading up to the currrent moment of action (which is where the story should begin). This is a classic newbie mistake.

Just get to the story - any important details about a character's life can be conveyed through dialogue, actions, and some exposition later on.