This is a topic I think you're fond of, expecially if you love the manga style. I sometimes do portfolio review or I'm questioned about becoming proficient in manga style. Maybe you noticed that there's a big difference between skilled manga artists and many who keep a ... wannabe style.
I'm going to tell you the secret. 😉
Maybe your passion for mangas started when you were around 13, or even earlier. Since then, you drew your favourite manga characters and draw in manga style only, with the goal to achieve a professional level.
But, along this path, the basics of drawing get discarded: the manga style is nothing but a veil over them. If you don't master the foundamentals, the veil won't stand up on its own.
The secret to achieve professional level in manga is not to burn steps, and learn anatomy, perspective and shading first.
Once used to "normal drawings", adding the manga style will become so simple you'll be surprised.
Also, it's important to study the Japanese culture to understand why some things -like noses- are drawn in a certain way. But I'd rather talk about this in the future.
Meanwhile, try to set the manga style aside and focus on mastering anatomy. To help you, here's a checklist for human body proportions. This will help you to begin.
If you wish, feel free to send me your questions and progresses!