The hair should be a few millimetres long at least, otherwise the wax can't get a grip, so to speak. Its a couple of week's growth.

If you have a low pain threshhold, take a nurofen an hour or so before your appointment. Otherwise, you'll be fine.

Suggest you tell the therapist you haven't had it done before and she will talk you through it. Its best to pull the skin taut for the underarms (less painful), but your therapist should tell you what to do.