Water samples should be taken from 18" below the surface of the water. If the pool has just been opened, the water should be circulated for at least 24 hours before being tested. If you get a chlorine reading higher than the max on your test kit, wait until the chlorine level comes down before testing the other elements of your water chemistry. Find a clean container that has never held other liquids than water(do not use soda bottles, shampoo bottles,  or food jars. Bring at least 16 ounces of water if possible. You can use a clean ziploc bag in a pinch. When you arrive at the store, ask for a Gallagher Pools and Spas sample bottle. Make sure your name and phone number is written on the front of your bottle either on the bottle or on a piece of masking tape. Some customers affix an address label with their name on it to the front with their phone number.