How do I come out ?

Say nothing.....finish school, be on your own, support yourself. If you still want to tell her, than it would be ok. You are in the damn south, full of hate.

Here's an idea: DON'T. 

The bigger question to ask is whether or not you should come out.  It entirely depends on how homophobic your family is, or your friends, or other people in your environment.  You're still living at home and dependent on your family, so if they are homophobic and would not be accepting then they may do anything that would make your life a total hell until you're able to move out.  If they are open minded and accepting then great, you might be able to come out and then could work on the "how" portion of it.  But if you think you could be disowned, kicked out, abused verbally or physically, forced into "conversion therapy", or any such horseshit, you really would be far better off keeping your sexuality private until you're old enough to get a job, gain independence, and move out on your own.  At that point there'd be nothing they could legally do to you that would actually harm you.

You might be able to come out, even if you live in Alabama or whatever. It's 2020 and it shouldn't be an issue anymore. 

Wait til your 18 if you don’t think she’ll be supportive 

She probably already knows.