I have mine done horizontally and vertically. I did the verts myself. Depending on size of nipple and how deep the piercing is done will impact how much it will hurt. As a piercer, I can tell you pain is relative and varies from person to person. You should expect some pain, if nothing else a pinch. Don't tense up and try to relax, it makes everything easier.
Make sure the piercer explains all proceedures to you and that everything is kept in sterile and individual packaging and opened in front of you. They should mark spots on your nipples to assure they allign correctly. They should be pierced with a circular or horseshoe ring, never a barbell.
For cleaning, wash with warm water and clean off any crust from the jewelry. It is important to do this so that the crust doesn't get inside the hole and tear or infect the piercing. Wash with warm water again and use a mild liquid antibacterial soap (I like Dial). Make sure the soap doesn't have moisture beads or strong scents. Once you wash the area, gentley move flip the jewelry up and then down again while you rinse off the soap. If you notice a green, yellow or brown discharge, you have an infection and should see a doctor. An additional option to help heal and to reduce soreness is sea salt soaks. This can be found and health and whole food stores. Mix the sea salt with warm water so that the solution tastes like tears. Either soak paper towel and hold on the area, or fill a shot glass and hold it over your nipple.
Healing should take 4-8 months even a year at most. Initial healing is 4-8 weeks. At that point, you can gently touch and play with them. Try to avoid changing jewelry for at least the minimum of that time frame. Everyone is different.
Be sure to clean them twice a day. Avoid using table salt, epsom salt, Bactine, alcohol and peroxides. Those solutions can dry out skin and make healing worse or painful.
Good luck to you
If you have more questions, message me.