Be wary of filling the first chapter(s) with back story. If you've not established your characters so the reader can relate to them from the outset, you risk losing them. If you feel you must fill in the back story, consider starting your story earlier.
Some advisers caution against having any back story at all. In reality unless your character appeared fully formed out of thin air, they have had a life before the story begins. Even for a baby, something brought her parents together to create her. So there is always a back story of some kind. But, is it important to the current story and situation? If not, leave it out.
One technique for beginning authors is to write your story where you think the beginning starts and then judge whether you can skip that chapter(s) to where the action really starts. This depends on the kind of book you're writing, of course.
If you determine your backstory is justified, don't tell it, show it in a complete scene. You'd be surprised how few sentences can construct a scene.