Hi there and welcome to WHL!
A positive ANA does not mean that you have Lupus. Many people have a positive ANA that don't have Lupus or another AutoImmune Disease (AI Disease). There is actually no single test for Lupus. A lupus diagnosis is made when a combination of different bloodwork and symptoms come together and there is nothing else to explain them (there is actually a "sticky" in one of the sections called "criteria for diagnosing Lupus").
When Lupus or other AI diseases are suspected and an ANA test is run. If it comes out positive it tells the doctor that they may want to do some looking in that area. It does not, however, diagnose any disease since even healthy people can have a positive ANA at certain times in their lives. Sometimes it can be something as simple as an infection in your body although it does usually indicate that your immune system is misdirecting its efforts.
the ANA test is simply a starting point for doctors. If you are having health issues they will most likely begin looking at your auto immune system for answers but that will require many other tests and an evaluation of your symptoms
hope that answered your question