An interval is the distance between any two notes. If the space between the two notes is the same, for example, if three spaces are between the two notes, or four spaces between the two notes, then this interval will sound the same.
The Most Widely Used intervals
To aid in sight-reading, you should memorize the way intervals sound. The trick to starting this, is to think of songs which have these intervals in them. Here are some songs which I use for these intervals -- some of them are hymns, which you may not know, so try to find your own songs to help you.
major second - easy - just a scale between Doh and Re.
minor third - "Oh Can - a - da" (between Oh, and Can)
major third "A-las, and did my Saviour bleed"
perfect fourth "Rise Up oh Men of God"
perfect fifth "The Last Post"
major sixth "St. Elmos Fire"
minor seventh "Star Trek, the Original Series Theme"
major seventh (I don't have a song for this one)
octave "Let it Snow" Well the Weath-er out-side
Make it Your Own
Again, come up with your own melody-pieces which contain these intervals, and try to memorize them so that when you hear the interval, you know what it is instantly.