rotate towards touch position around Z

[jister]

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

using UnityEngine;
using System.Collections;
 
public class Controller : MonoBehaviour {
 
    public float speed;
    public Vector3 axis;
    
    
    void Update () {
        
          int i = 0;
        while (i < Input.touchCount ) {
           
            if (Input.GetTouch(i).phase == TouchPhase.Began){ 
                
                Vector3 touchPosition = Input.GetTouch(0).position;
                Vector3 relativeTP = touchPosition - transform.position;
                Quaternion targetRotation =  Quaternion.Euler (0,0,relativeTP.x + relativeTP.y );
                transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, Time.deltaTime * speed);
 
            }
 
            
            ++i;
        }
    }
 
    void FixedUpdate () {
     float autospeed = 5f;
        transform.Translate(axis * (Time.deltaTime * autospeed));
        
    }
}

[jister]

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