Tubes almost always have the valve molded into the tube, not able to remove or reinstall. Tubeless set ups do have the valve as a separate item and they are able to be removed and reinstalled into the rim, there being no tube.

Generally the tube's size is the same as the tire's. The tire should have the diameter and width listed on it's casing. If in doubt bring the tire/wheel/old tube (assuming it was a good fit) to the LBS and they will guide you. Andy