How to cure a slice?

I'm a youth golfer. 14. And even worse, left handed. When I'm playing matches/tournaments, I always think after the round, "What do I need to improve on?" One thing that's always been on that list; my driving. After a drive, I am zoned in. That is, if I hit the fairway. If I get a hold of a drive, it can go 260-270. But I always slice. I've tried everything to fix it. Nothing works. So Reddit, does anyone know any good drills or techniques to help eliminate this so dreaded slice?

-Cheers.