Welcome! First read the Index, it will have loads of information on the basic techs and movement options that are used in this game competitively, It will also have links if you want to play online, a good option for playing people that are already seasoned at the game and will probably accelerate your idea of what can be done in the game, albeit by getting rekt most likely, but don't let that discourage you.
Anyways basically you're going to want to learn to z-cancel, short hop, and pivot. those are the basic techs, pretty much need these no matter what characters you're using (though since you're planning on maining ness up air aka uair and down air aka dair do not need to be z canceled.
Since you're maining ness he has a character specific tech called DJC which he shares with Yoshi, but their DJC's are different. You should watch videos of people who are good with ness and see how they use their first and second jump to move. I'd recommend watching people like Firo, Peek, Frogles, Nangoku at apex, and I don't really know who else lol.
Also just some advice, Don't get grabbed, Ness isn't really good off stage, so really make an effort if you start playing online to not get grabbed as ness.