ive gone to state and regional athletics before and got offered a sports scholarship for athls/soccer, so let me tell you how most people i have trained with train
i dont know what school you go to or what your eventual goals are so ill make my info generic
firstly - you should pick an event or two which will be your main ones (they dont have to be related) for me it was 100m and high jump - judging by the OP yours would (probably?) be 400m and long jump. try to get competitive times/distances in these, hopefully you have a natural ability in some; just try every event and see which ones you excel at, then train in those, most people end up favouring one over the over.
for jumping, strong legs are important, for long jump speed is also important because if youre fast, odds are you have strong legs, and thus momentum and leg strength will help, technique is also important but a bit less so than high jump
advice: do leg day at the gym, practice, if you can join a sport that does kicking (soccer/footy) do that, but its a bit late in the season now
i think that with the 100m you can get your starts good, and refine your ability, but some people are just insane 100m sprinters from the get go, and when they get training; they annihilate the competition, so if you arent naturally fast at 100m i'd probably drop it, depending on your zone/region/state(obvs vic?), odds are you will end up having some random who runs funny but wins by like a metre, and if you get to state, you get those crazy fast kids, and one of them will have training and will win by like 5 metres so yeah, a way one of the seniors at my athletics club put it to me was "if you cant top your year level at school with no training, you probably wont be able to get far with training" anecdotal evidence to support: two kids at my school have done athletics outside of school for 2+ years (i know at least 2), they took it seriously at our school carnival wearing spikes and compression pants and everything, one came third, the other didnt place, alot of people at the first level of interschool athletics (zones) think theyre all going to win 100m because theyre the best at their school, but they dont understand the level of competition
400m is a good event to try hard in because you can improve fitness with training, so if you run tonnes of 400m sprints, you will get good at it, but seriously get really fit for 400m, and your leg strength training in long jump will probably complement that.
weekly training for a kicking sport tends to help with everything, but its a bit late in season to join anything now
best of luck !