Take a look at the code from the manual.

using UnityEngine; 
using UnityEngine.UI; 
using System.Collections;
public class ClickExample : MonoBehaviour 
{ 	
  public Button yourButton; // Put your button in here
  void Start () 
  { 		
    Button btn = yourButton.GetComponent<Button>(); 		
    btn.onClick.AddListener(TaskOnClick); 
    //When the button gets clicked/pressed it will call TaskOnClick() function below	
  }
  void TaskOnClick()
  { 		
    Debug.Log ("You have clicked the button!"); 	
  } 
}
I hope this helps.
I've struggled with this same topic a while ago 
and this helps