rotate towards touch position around Z

hey, i'm making my first android game and got a bit stuck at what i tought was going to be fairly simple.

i got a game object that always moves forward and i want it to rotate smoothly towards the touch position so that it makes a nice curve.

I'm only coding for a good 6 months learning from a friend at work bit by bit so i still get a bit confussed with all this quaternion stuff.
anyway this is what i got so far, it does sometimes what i want

Code (csharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class Controller : MonoBehaviour {
5.  
6.     public float speed;
7.     public Vector3 axis;
8.    
9.    
10.     void Update () {
11.        
12.           int i = 0;
13.         while (i < Input.touchCount ) {
14.            
15.             if (Input.GetTouch(i).phase == TouchPhase.Began){
16.                
17.                 Vector3 touchPosition = Input.GetTouch(0).position;
18.                 Vector3 relativeTP = touchPosition - transform.position;
19.                 Quaternion targetRotation =  Quaternion.Euler (0,0,relativeTP.x + relativeTP.y );
20.                 transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, Time.deltaTime * speed);
21.  
22.             }
23.  
24.            
25.             ++i;
26.         }
27.     }
28.  
29.     void FixedUpdate () {
30.      float autospeed = 5f;
31.         transform.Translate(axis * (Time.deltaTime * autospeed));
32.        
33.     }
34. }

 

come on it can't be that hard for one off you guys to look at this and see whats wrong... ;-)