How can I make a block placement system for a space survival game. (Minecraft-like)

I'm very new to Unity and I'm for practice making a small survival game.
I have found block-placement code on these forums somewhere, but this is VERY buggy.

This is what I currently have:

Code (CSharp):

1. Ray ray = camera.ViewportPointToRay(new Vector3(0.5F, 0.5F, 0));
2.         BoxCollider box = hit.collider as BoxCollider;
3.  
4.         if (Physics.Raycast(ray, out hit))
5.         {
6.             print("I'm looking at " + hit.transform.name);
7.             box.GetComponent<Renderer>().material = Looking;
8.  
9.             if (Input.GetMouseButtonDown(1))
10.             {
11.                 Destroy(box.gameObject);
12.                 print("Removed block");
13.             }
14.  
15.             if (Input.GetMouseButtonDown(0))
16.             {
17.                 Vector3 V = new Vector3(Mathf.Ceil(hit.point.x), Mathf.Ceil(hit.point.y), Mathf.Ceil(hit.point.z));
18.                 Instantiate(prefab, V, box.transform.rotation, parent.transform);
19.                 print("Block Added");
20.             }
21.  
22.             else
23.                 print("I'm looking at nothing!");

Sometimes it even places the block inside the other ones.

Could someone please fix this code for me?
Thanks in advance.