You can either buy some rubbing compound (like Meguiars Diamond) and buff it yourself by hand, or you can take it to a detailing shop and have them do a nice detail on it. Depending on how deep the scratches are, just a little detailing could make it as smooth as glass. If they're REALLY deep, then you might need to take it to a body shop, and have them repainted. If it's actually to the point where it's damaged the plastic, I'd just get a new one because you'll basically never get it to look perfect without fillers and things that you just don't want to use.