You can choose not to copy anything. In that case, Sculpting Steel stays on the battlefield as an artifact that doesn’t do much of anything.

If the chosen artifact is copying something else (for example, if the chosen artifact is another Sculpting Steel), then your Sculpting Steel enters the battlefield as whatever the chosen artifact copied.

Any enters-the-battlefield abilities of the copied artifact will trigger when Sculpting Steel enters the battlefield. Any “as enters the battlefield” or “enters the battlefield with” abilities of the chosen artifact will also work. For example, if Sculpting Steel copies an artifact creature with sunburst, the copy will get +1/+1 counters based on the number of different colors of mana used to pay Sculpting Steel’s total cost. If Sculpting Steel copies a noncreature artifact with sunburst, the copy will get charge counters based on the number of different colors of mana used to pay Sculpting Steel’s total cost.