HELP!

hi! my daughter is 3 next month and i still cant get her to go on the potty! as soon as i take her nappy off she starts holding herself and trys not to go and if she does go on the floor she starts hisaricly crying even though im reasuring her its ok???? when she did go on the potty the one time she started to pee and tryed stopping herself and crying! im trying to make the expeariance as happy and calming as possible for her, but no use! has anyone else expereanced this ???? =( and what can i do ???