(To do this tutorial you need to have learned the note names.)
An interval is a distance between two notes.
Intervals have names like
4th, so C to D is a 2nd, C to E is a 3rd, C to F is a 4th, and so on.
C to D is the same distance as D to E, so D to E is a 2nd as well. As is G to A.
Note that an interval is not a singe note. C to D is a 2nd, but D by itself is just a note. This is where the distance analogy is useful. London to New York is a distance, but New York’s just a city.
Now that you know what an interval is, go and learn your first one: semitones.