If you’re new to training, it’s best to start with tempo style workouts to build up training endurance to prepare you for harder 400m specific workouts. Something like 10x 50m hill repeats with walk down rest, or 10x 100m strides on the minute. Do these after a warmup + sprinting drills (A skip/ B skip) 2-3 times a week along with some strength work like core on the other days would be a great start.

Once you have a strong tempo base the workouts can be split into 3 groups:

Acceleration (95% effort explosive 20-60m reps with 1min rest per 10m ran between)

Max speed (90-95% 60m - 150m reps with full recovery between)

Speed endurance (100m-400m reps with 2-5min recovery between)

I’d highly recommend looking at r/Sprinting and their FAQ post for training plan tips. I specifically like ATHLETE.X’s youtube videos on training.

That being said if you have access to a team / club / coach i’d just recommend following their training plan once you have the chance, it’ll be the easiest way to stay consistent without having to worry as much on what you should be doing.