Be careful with the “t” at the end of “don’t”. You should begin to form it by placing your tongue in the correct place, just behind your top teeth, but not touching, but don’t release it. Instead, make a slight stop, then let the air through as the /s/ sound, so your tongue stays in position through both sounds. You might need to practise this.
The phrase expresses surprise, but be careful, because of that, the key should be high. If you use a low key, it will mean “I’m not surprised at all.”, but in a sarcastic way.