An interval is simply the distance between two notes. We can divide music into sounds for example Major, minor, diminished and augmented, each sound has it's own particular characteristics (or flavour) which we use to our advantage when creating music.
For now let's focus on the Major scale as it's used extensively in western music, we could also say that the Major scale is the basis for the majority of western music regardless of genre. The Major scale contains 7 notes (8 including the octave ) and each one of these notes is also an interval. The important thing to note here is that we are looking at each interval from the root note, in this case the root note is C.
The C Major scale is C D E F G A B (C), lets look at the intervals!
C - D = Major 2nd
C - E = Major 3rd
C - F = perfect 4th
C - G = perfect 5th
C - A = Major 6th
C - B = Major 7th
C - C (octave)
As we touched on earlier as a musician it's equally if not more important it listen to and hear the quality of each interval, as each interval has it's own quality or sound. I use other recognisable sounds to reference the sound of each interval.
C - D = Major 2nd (Happy Birthday)
C - E = Major 3rd (Oh When The Saints)
C - F = perfect 4th (Here Comes The Bride)
C - G = perfect 5th (Twinkle Twinkle Little Star)
C - A = Major 6th (Go West)
C - B = Major 7th (Take On Me)
C - C (octave) (Somewhere Over The Rainbow)
These are not the only sounds that you can use, there are many so why not find your own? Oh and lastly SING! The best way to get used to intervals is to sing or hum them. You don't have to have the best voice in the world you don't even need to be a singer, give it a go.