GetComponent usage

 public class startingForce : MonoBehaviour
{
    public float yval;
    Rigidbody thisRigidBody;
    // Start is called before the first frame update
    void Start()
    {
        thisRigidBody = GetComponent<Rigidbody>();
        thisRigidBody.AddRelativeForce(0,yval,0);
    }

    // Update is called once per frame
    void Update()
    {
        
    }
}

Like this! ^
here we use GetComponent to look for the Rigidbody component attached to the same GameObject

Note the use of < to surround > the component type and how it is followed by the ( ) <-something I keep forgetting. Also note how we make a variable of the same component type equal to it, so we can then refer to it.