Please Help. How to add score on collision mobile?

Hello everyone. I am creating a 2d Mobile, touchscreen game for unity. I would like to have a UI score increase every time I touch the object. When I touch the specified object there is a collision but I want to enable a score for when these objects collide when I touch them. I will select the best answer.

@putback This will work as a basis for what you want:

    using UnityEngine;
    using System.Collections;
    using UnityEngine.UI; //Required when using UI elements in the script.
    public class ExampleScript : MonoBehaviour {
        public int currentScore;
        public Text displayScore;
    	// Invoked on collision
    	void OnCollisionEnter2D (Collision2D coll)
            if (coll.gameObject.tag == "Player")
                //Add 20 points each time the player collides 
                // with the object this script is attached to
                currentScore += 20;
    	// Update is called once per frame
    	void Update ()
            displayScore.text = currentScore.ToString();

@Ben Stoneman . I need help, for some reason the score count is staying at 0 and I keep getting the error: Null Reference Exception: Object reference not set to an instance of an object.

on lithis line: displayScore.text = currentScore.ToString();