Hi how do I rotate an game object, like a car?

You can learn about ackerman steering from here: https://en.wikipedia.org/wiki/Ackermann_steering_geometry

I attached a dummy pivot gameobject for rotation since rotation should be around back wheel center. I also added sample script for steering. Just add to an cube and test.

 using UnityEngine;
 using System.Collections;
 
 public class SteeringTest : MonoBehaviour
 {
     public float backWheelDistance = 1;
     private GameObject dummyPivot;
     float turningCenterDistance = 5;
     void Start()
     {
         dummyPivot = new GameObject("dummyParent");
         dummyPivot.transform.parent = this.transform;
         dummyPivot.transform.localRotation = Quaternion.identity;
         dummyPivot.transform.localPosition = Vector3.zero;
         dummyPivot.transform.parent = null;
         this.transform.parent = dummyPivot.transform;
         this.transform.localPosition = new Vector3(0, 0, backWheelDistance);
     }
 
     void Update()
     {
         if (Input.GetKey("left"))
         {
             Vector3 turningPivotPoint = dummyPivot.transform.TransformPoint(new Vector3(-turningCenterDistance, 0, 0));
             dummyPivot.transform.RotateAround(turningPivotPoint, -Vector3.up, 20 * Time.deltaTime);
         }
         else if (Input.GetKey("right"))
         {
             Vector3 turningPivotPoint = dummyPivot.transform.TransformPoint(new Vector3(turningCenterDistance, 0, 0));
             dummyPivot.transform.RotateAround(turningPivotPoint, Vector3.up, 20 * Time.deltaTime);
         }
     }
 }

Hope that helps!

[1]: /storage/temp/68924-488px-ackermann-turningsvg.png