NullReferenceException: Object reference not set to an instance of an object

Hi everyone and i need your help. I don’t understand what i need to do. Can you look at my script and help me please.

using System.Collections;
using UnityEngine;
using System.Collections.Generic;
using UnityEngine.SceneManagement;
using UnityEngine.UI;

public class exam : MonoBehaviour {
	public string a;
	public InputField imf;
	public void Start(){
		string a = imf.text;
	}
public void OnClickStart()

{ 
		if (a == "Na") {
			SceneManager.LoadScene (0);
		}
	}
}

If the exception is pointing to like 11 then you have not set imf in the inspector.

Ryuuguu is probably right, but in future you should post the entire exception, which will tell you the line number where the error occurs.