An object reference is required to access non-static member `UnityEngine.Component.GetComponent(System.Type)'

Hello, i want to play and stop my music in my game, i don't have any problem when I do stop, but when I do play, appears this message "An object reference is required to access non-static member `UnityEngine.Component.GetComponent(System.Type)' "

this is my code:

using UnityEngine; using System.Collections; using UnityEngine.UI;// we need this namespace in order to access UI elements within our script

public class audio : MonoBehaviour {

 public bool soundoff = true;
 public Button botonSoundOn; 
 public Button botonSoundOff; 
 

 public void start(){
     botonSoundOn = botonSoundOn.GetComponent<Button> ();
     botonSoundOff = botonSoundOff.GetComponent<Button> ();

 }

 public void PararSonido(){
     Camera.main.GetComponent<AudioSource>().Stop();
     soundoff = false;
 }
 public void PonerSonido(){
     Camera.GetComponent<AudioSource>().Play();

     Debug.Log ("entras");
     soundoff = true;

 }

}

Please help!