I made a cube fly around like a spaceship with this script:

function FixedUpdate()
{rigidbody.velocity = Vector3(
        Input.GetAxis("Horizontal") * speed,
        Input.GetAxis("Vertical") * speed,
        0,
   );
}

This makes the spaceship fly around on the xy plane. You may need to change the speed depending on the mass of the spaceship. Also, make sure the the Horizontal and Vertical axes are set to wasd keys and the rigidbody is not affected by gravity.

I hope this helps.

If you want to move the object using applied forces, it might look something like this (untested):

function FixedUpdate()
{
    rigidbody.AddForce(
        Input.GetAxis("Horizontal") * forceMagnitude,
        Input.GetAxis("Vertical") * forceMagnitude,
        0
    );
}