You don't have to dance at the prom.
Watch the people at the prom who are dancing. Few will be good, and those that are best will have all of the spotlight. Let them have it. Say to them, "Awesome dancing!" and admire them from afar.
Dance in front of a mirror. Good god, make sure no one films this, or you'll be scarred for 20 minutes after watching it on Tosh.0. Count along with the music, and shift your weight to your left foot on the first beat, and to your right on the second. Congratulations, you've done the Merengue.
When you master weight-shifting, vary this by picking up the unweighted foot and moving it forward or backward (it really doesn't matter which direction at first) so that you slowly pivot. Vary which foot pivots as well, but keep the weight shifting constant. Congratulations, you've done a turn.
Tips: don't move your arms, particularly if you have a tendency to mime jogging movements. Exceptions: if you're literally miming jogging for comedic effect. Instead, with your hands at your side, let the weight shifting move them for you.
After you've mastered the shifting and pivots, start shifting your weight from front to back (toe to heel). With a slight bend in your knee, contract your back so that your chest moves forward about an inch. Contract your abs so that your chest sinks back. Sink back on beat one, chest forward on beat two. Keep this movement small, so that your pelvis doesn't get too engaged by the rocking or you'll look like you're humping your imaginary friend.
Tips: alternate the forward/backward movement with the side to side weight shifts. Remember, back or left on beat one, forward or right on beat two. Try to do both at once (with your feet about 10 inches apart width-wise and your right foot ahead of your left about 4 inches). Keep your arms low. Only engage your pelvis in a forward/backward movement if you intend to mime humping an imaginary friend for comedic effect.
About comedic effect: if you are in fact a funny person, you'll know that humping an imaginary friend is not funny. It's sad. Instead, hump an actual friend. Remember, back on one, forward on two. Keep your knees slightly bent so that your pelvis can thrust freely. There should be some impact against your friend (depending upon if you're the giver or the taker, from your front or your rear). Doing this longer than eight beats is not funny. It is also sad. Humping should last for no fewer than two beats, no more than eight. That is essentially your comedy window.
At the end of eight beats, lobbeth the Holy Hand Grenade of Antioch at thine enemies.