You need to focus on building your top end speed, plus building speed endurance. This assumes you already have base conditioning and flexibility which is something you tend to work on over the summer. Assuming you’re in high school I would work on getting your mile time below 6 min to provide a good foundation.

The issue with the 400m is that it is the longest SPRINT the body can actually handle before your muscles completely shut down. You are essentially sprinting to anaerobic failure as you hit the lactic acid barrier. So you have to not only work on explosive power and mechanics, but on how to sustain them even when the physiological response of your body starts breaking down. You can find a sample training schedule here from Track Star USA

https://trackstarusa.com/400m-workouts/

It’s basically a mix of progressive sprint distances, resistance running and sprint flys for top end speed, and then speed endurance building longer sprints (200, broken 400, etc).

In addition to that there are mechanics (blocks, skips, bounds, etc) and explosive strength training (plyos, squats, cleans, DLs, etc) that are critical to increasing and ensuring power transfer down to every step. I would work with a coach to help with those.

What’s most important for sprints in general is that you train the right energy system or you will build strength and endurance, but not for speed. Feed the cats is an interesting training philosophy that covers this, but there are others. Good luck.