Depending on what kind of "piece" you are working on, here are a few ways to approach your first day:
Use resources like Adam Savage's One Day Builds for inspiration on completing a project from start to finish in a single session. For Complex Puzzles: 1.Day One
Focus on completing just one section or "piece" of the larger work to build momentum. For Physical Builds (PC or DIY): Depending on what kind of "piece" you are