The length of your hair is one of the most important factors when scheduling your wax. A minimum of two weeks grow out from your last shave is ideal, (about 1/4") but more is certainly fine. Any less and the process could be more painful, with poor results.
Aside from growing out the hair, you could gently exfoliate the area to be waxed the day or two before. This is especially helpful if you are someone who is prone to ingrown hairs.
After care will be discussed at your appointment as it can vary depending on your skin and hair type.
Discomfort is very subjective and varies from person to person. Your hair and skin type play a role in this, but having an experienced waxer who knows what they're doing can greatly decrease the amount of pain you experience. Stimulants such as caffine and alchohol can increase sensitivity as can pregnancy and menstration. It's common to feel like you have a mild sunburn anywhere from 1-48 hours after your first wax, but with consecutive waxes clients typically feel little to no pain post wax.
Waxing is not a good idea if you are currently taking antibiotics or acne medications. This is particularly important with facial waxing as these can make you more prone to skin removal. If you've been on Accutane, consult with your doctor before starting waxing, but it's typically suggested to wait anywhere from 6-12 months after stopping your medication.