Before adjusting expo/rates etc., you need to make sure the servo is set up properly in the car.

1. Make sure it is centred and the linkages are geometrically even. Use sub-trim instead of trim to get this right (I will use Futaba naming, not sure what Spektrum call it)
2. Set the end-points (EPA). Do this with the dual rate set at 100% (no more no less). You should be hitting full lock, but not straining the servo.
3. On the track, set the trim for straight line running
4. Dual-rate limits overall steering
5. Expo can be used to make the steering less responsive near the centre
6. Speed will slow the servo down

4/5/6 should all be used in moderation - go too far and you will make the car safe but slow. Try and be smooth with your inputs instead of relying on the radio.

If the car is still very twitchy, it's going to be a build or setup issue.