The best website for ear training is music theory.net. We are training our ears to hear different intervals – the spaces between two notes. In the exercises, find interval ear training, and customize the exercise to only have unison, octave, major 3rd, perfect 4th, and perfect 5th.
After you get 80% from 30 questions, add minor 2nd, minor 3rd, and the Tritone.
This takes patience. You will improve the more you listen.