How to make objects collide

I am currently making a game like crossy roads. I have the prefabs and have added colliders to all of them, but when I want to play the game, the player just goes straight through the tree/car. What did I do wrong? Another problem is that my playerHealth script does not work form some reason. Here is my code :

 using UnityEngine;
 using System.Collections;
 
 public class PlayerHealth : MonoBehaviour
 {
     public GameObject playerDead;
 
     Vector3 playerPos = new Vector3(0, 0, 0);
 
     float posX;
     float posY;
     float posZ;
 
     void OnCollisionEnter(Collision collision)
     {
         posX = transform.position.x;
         posY = transform.position.y;
         posZ = transform.position.z;
 
         if (collision.gameObject.tag == "car") 
         {
             Destroy(collision.gameObject);
             playerPos = new Vector3 (posX, posY, posZ);
             GameObject playerDeadIns= Instantiate (playerDead) as GameObject;
             playerDeadIns.transform.position = playerPos;
         }
     }
 }

When I come in the way of a car, the car just goes through me and not$$anonymous$$ng happens to the player. What is wrong?