This is probably the easiest way to get started learning intervals: pick a handful of songs you already know (folk songs, nursery rhymes, Christmas carols, TV theme tunes, etc.) which have a melody starting with the interval you're trying to learn. For example, learn a perfect fifth with the Star Wars theme.
How long does it take to learn intervals?
If you want to learn how to identify intervals and scales in music, it could take anywhere from 4-8 months because there are a lot of different intervals to practice (learn about intervals here in this music theory lesson).
What is the fastest way to recognize intervals?
Since we identify intervals by counting from the bottom note, it makes more sense to read the intervals (or chords) from the bottom note. For example, if you have this interval, look at the bottom note, which is E, and the interval, which is a fourth. So you would play E with the note up a fourth, which is A.