Practice laughing at the lines at home in front of the mirror, and think, "How would I feel if someone laughed during MY lines?" It will cure you, you know you are a professional, and must stay in character. Otherwise, when you laugh on stage inappropriately, you are just embarassing yourself, not the person you supposedly laughed at. You can do it, it will make you a much better actress.
AND YOU KNOW WHAT??
U WILL BCOME AN EXCELLENT ACTRESS IF U WILL MARK MY ANS AS BEST...MY GOOD WISHES ARE WITH U...
-GOD BLESS U