If your character is a CharacterController, use the property isGrounded:

  public float speed = 6;
  public float jumpSpeed = 8;

  CharacterController cc;
  float vVel = 0;

  void Start(){
    cc = GetComponent< CharacterController>();
  }

  void Update(){
    // get movement direction relative to the character:
    Vector3 vel = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
    vel = transform.TransformDirection(vel)*speed; // convert to world space
    // only jump when the character is grounded:
    if (Input.GetButtonDown("Jump") && cc.isGrounded){
      vVel = jumpSpeed;
    }
    vVel -= gravity * Time.deltaTime; // apply gravity
    vel.y = vVel; // combine vertical and horizontal velocities
    cc.Move(vel * Time.deltaTime); // move the character
  }

But if your character is a rigidbody, you must check if it's grounded - a raycast downwards is a good alternative:

  public float speed = 6;
  public float jumpSpeed = 8;

  float height; // raycast length 

  void Start(){
    rigidbody.freezeRotation = true;
    // calculate height from ground plus a little margin (0.2)
    height = collider.bounds.extents.y + 0.2f;
  }

  void FixedUpdate(){
    // get movement direction relative to the character:
    Vector3 vel = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
    vel = transform.TransformDirection(vel)*speed; // convert to world space
    vel.y = rigidbody.velocity.y; // keep rigidbody's vertical velocity
    // only jump when the character is grounded:
    if (Input.GetButtonDown("Jump") && Physics.Raycast(transform.position, Vector3.down, height)){
      vel.y = jumpSpeed;
    }
    rigidbody.velocity = vel;
  }