define a public variable of type GameObject in your component (script) and then in the inspector drag a prefab on it and you can instantiate it.

public class script1 : MonoBehavior
{
public GameObject ball;
void Start ()
{
GameObject myball = (GameObject)instantiate (ball);
}
}

when you attach this small script to a gameObject it will add script1 component to it's inspector and the script1 component has a slot called ball. you should drag a prefab to it and then the code will instantiated that dragged prefab.

public GameObject obj;

void Start () {
    GameObject myPrefab = (GameObject)Instantiate(obj);
}

Use Prefab in your Game Here is the tutorial for that . http://lnkd.in/J6ufVr . Here is demo and code for Prefab instantiate so download and try to undestand i hope u will get the solution ..