This simple web app will allow you to develop your listening skills. Intervals are named after their pitch difference relative to the major scale. 
The intervals in the Major scale would be as follows: Unison(0), M2(1 tone), M3(2 tones) P4(2.5 tones), P5(3.5 tones), M6(4.5 tones), M7(5.5 tones), P8 or Octave(6 tones).
The distance from the first note(doh) to the third(mi) would be a M3. From the root(doh) to the fifth(soh) is p5. Major intervals made a semitone smaller by lowering the upper note become minor and perfect intervals become diminished. 
Melodic intervals are played consecutively and harmonic intervals are played concurrently. This app will play two notes randomly and you will then tryy and name the intervals.
Select either Melodic or Harmonic to continue.