Do you mean that you will be tested on your aural skills? SIght singing alone does not directly require the identification of intervals, even though that skill improves sight singing. I will assume that you want to be able to hear an interval, and then name it. I will also assume that you do not have perfect pitch.
First of all, be sure that you understand intervals on paper. If necessary, review how to identify and write specific intervals on paper (e.g. given a G, write a note that is a perfect 5th higher). If that is not a problem, then try the following. Even if you do struggle with theory, the following suggestions might help.
One common method, which is not really the best way, but can help in a pinch, is is to find songs or pieces that start with the various intervals. For example, Amazing Grace starts with an ascending perfect 4th. The first movement of Spring from the Four Seasons starts with an ascending major 3rd.
Although this works for some intervals, it is hard to find pieces that start with intervals such as a tritone. Additionally, this method is more of a crutch than a way of fully understanding intervals. As I said, though, it can help sometimes.
Another way is to work from the major scale. The tonic forms a major or perfect interval with every other degree in the scale. I will describe how to identify major and perfect intervals by ear.
When you hear two notes, sing a major scale, ascending, starting on the lower of the two notes, until you reach the second note. If you are given C and G, sing C D E F G (major scale). Since you have sung five notes of a major scale, the interval is P5 (perfect 5th). Suppose you have D and F♯. Sing D E F♯. Since you have sung I II III, the interval is major 3rd. If it is a large interval such as a 7th, it can help to sing an octave first; you will then realize that the interval is one degree smaller than the octave.
For minor, diminished, and augmented intervals, you just have to realize how they relate to the major and perfect intervals. First, find the nearest perfect/major interval, and then decide decide whether the interval you are hearing is bigger or smaller than that. For example, suppose you hear A and C. The major scale gives A B C♯. C is a semitone LOWER than C♯. The interval is smaller than the perfect interval, so it is minor. Remember that when perfect intervals are made smaller or larger they become diminished or augmented.
After practising this way for a while, you will no longer need to sing all of the notes in between. To get practise, try one of these websites:
http://www.good-ear.com
http://www.musictheory.net/exercises (also has written interval identification exercises)
http://www.teoria.com/exercises/index.php