Learn the American English Pronunciation of TH. There are two TH sounds:
1) Soft TH: this sound is "voiced." It vibrates a little bit. It is mostly used in common words, and the tongue is in the same position as Hard TH.
2) Hard TH: this sound is not "voiced." It is made by putting the tongue between or just behind the teeth. The tongue presses and then moves away. The tongue is thickly pressed for a moment--the sound should not be the same as D.