Need help with adding ground detection to my jump. (c#) (3D)

As i wrote in the title i need with ground detection. I want to make it so that only can jump once and i know that you can use ground checking or raycast and i have tried both but i couldn´t get it to work.

My movement code*:*
public class Boll_kontrollLVL3 : MonoBehaviour {

	public float speed;
	public Text countText;
	public Text winText;
	public float jumpSpeed = 100.0f;
	private Rigidbody rb; 
	private int count;

	float movementSpeed;

	void Update()
	{
		float amountToMove = movementSpeed * Time.deltaTime;
		Vector3 movement = (Input.GetAxis ("Horizontal") * -Vector3.left * amountToMove) + (Input.GetAxis ("Vertical") * Vector3.forward * amountToMove);
		rb.AddForce (movement, ForceMode.Force);

		if (Input.GetKeyDown ("space")) {
			rb.AddForce (Vector3.up * jumpSpeed);
		}
	}


	void Start ()
	{
		rb = GetComponent<Rigidbody> ();
		count = 0;
		SetCountText ();
		winText.text = "";
	}

	void FixedUpdate ()
	{
		float moveHorizontal = Input.GetAxis ("Horizontal");
		float moveVertical = Input.GetAxis ("Vertical");

		Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical); 

		rb.AddForce (movement * speed);
	}

	void OnTriggerEnter(Collider other) 
	{
		if (other.gameObject.CompareTag ("Pick Up")) {
			other.gameObject.SetActive (false);
			count = count + 1;
			SetCountText ();
		}
	}

	void SetCountText ()
	{
		countText.text = "Cubes Collected: " + count.ToString ();
		if (count >= 12) {
			winText.text = "Level cleared!"; 
		}
		if (count >= 12) {
			Application.LoadLevel ("Roll a Ball 04");
		}
	}
}

helooo
i’m BR so my english are NOT perfect
sooooo…
for some time i had the same problem
buut…
exits OnTriggerEnter right?
so,exits also a OnCollisionEnter too XD
well
here is my ground detection system

void OnCollisionEnter(Collision any){
	if (any.collider.tag == "MultiversalCollidingInertObject") {
		onGround = true;
	}
}

hope i was helpfull,
byeeeeee