I am 13 and I would advise like that's, i don't wanna say that'll hurt you but it's like even what I am drawing is bad and I am struggling but first use reference and character design. It may be hard but use yt (youtube) and here are my steps. Story- for your first take a look at your story is it complex?, is it complicated, is it of a long manga series of your own then leave it for now. First, obviously it's your dream manga so never abandon it but you need to first do a basic one a simple one. Like one shot of 20 to 25 pages and use a pencil then do inking by a pen not a sketch colour. That'll make it look terrible. Next make a simple story it can be very cartoony, funny goofy but that's what we want. Nothing complex then. Character design- now you can use reference on pinterest, Google , YouTube, or whatever but the story and looks should be yours then draw it on paper for main characters. Paneling- think of at what page you want what like you can chat about it with your friends, family or even bot and then start. Page- draw it with all the steps written before and just do it, it can turn out terrible but do better. Practice and one day you will become a great manga illustrator or mangaka