How to create an Asteroids style moving in 2D mode?

I’m working on an asteroids style game, and I can’t figure out how to apply a forward velocity for the ship. So far I’ve done is a rotation but any form of forward thrust I can’t do.

This is my code so far:

    public float speed = 10.0f;
    public float rotationSpeed = 10.0f;
    // Update is called once per frame
    void Update()
        if (Input.GetKey(KeyCode.W))
            //not working
            rigidbody2D.AddForce(Vector2.up * speed);
        float rotate = Input.GetAxis("Horizontal") * rotationSpeed;
        transform.Rotate(0, 0, 0 - rotate);

For a start, use rigidbody2d.addrelativeforce as shown here: Unity - Scripting API: Rigidbody2D.AddRelativeForce
This will add force along the local axis.
This will make setting the direction of force easier.

This helped me find this: Unity - Scripting API: Rigidbody2D.drag which helped me slow down a RigidBody Underwater. Thank you!