Draw a manga ≠ Draw a face
I have seen many tutorials starting with “How to draw Manga” that are actually
“how to draw a straight face in manga style”…
Not quite the same.
A Manga requires a story, characters and drawing skills.
So this is more about 3 ways to start creating a manga.
1) Start with the character
One of the most common approaches to start a story (manga or not) is to draw and design an interesting Main Character that you like.
The biggest pitfall of this approach for manga artists is to focus too much on the character design and forget about making him/her interesting.
The backstory, the life goals, the conflicts, the good and bad sides, weaknesses, the choices made and awaiting for the Main Character.
Those are all good points to show in your story to begin or end with.
2) Start with idea/concept
Story writers obviously prefer this one. You come up with an interesting idea or concept.
Maybe by refreshing a stereotype story with a new twist (a battle tournament with national representatives to fight world wars), you can combine two different categories (like a detective story + fantasy world).
There are many ways but you have to try and go further than simply giving original names to characters, species and worlds to make the story stand out.
3) Start with the world/scenario
It’s more atypical, but you might find more inspiration for your plot development if you imagine the whole picture, the scenario in which your story unfolds.
What’s the main theme in your world?
Is it futuristic? A fantasy world with magic and monsters? A different world where all sorts of swords are the main thing?
There are classic examples about a type of people (Ninja, Pirates, Shinigami, Sayan, etc.) but you can find it anywhere else too (card games, pet monsters, being super-smart or crazy murderous, etc).
You probably pictured a few manga/anime with those themes because just like in marketing and business, they have carved out a niche or created one for themselves.
You don’t want to be generic. Yet another isekai fantasy battle manga with a slight non-memorable twist, or another shoujo love story, or another school life manga.