first of all its just a sparring match so your not in any danger of being seriously injured, you'll just be trying to find your range and get the feel of the ring. if you have a longer reach than your opponent, a significantly longer reach, then keep the jab popping without hesitence. im short and stocky so i barely ever have a reach advantage but i use a simple technique to put my opponent off balance "catching". the way it works is you wait for your opponents jab and push his left glove down with your right glove(if your both orthodox) a few inches and counter. this is not parrying because parrying is a much eggagerated version of this technique. how old are you?

anyway, good luck!!!!