If I had to judge this for a contest, I wouldn't bother taking it out of the envelope it came in.
I have a lot of criticisms for this comic if you're trying to shoot for "manga" quality.
1) Flat.
Everything looks flat. And I don't mean that because it's on a flat surface, but rather everything looks flat and devoid of depth, something that gives the illusion of a 3D object in a 2D plane.
This is prominent in the first page.
2) Speedlines.
The use of speed lines is to invoke a sense of an object passing by so fast that it looks a like a blur to the human eye, as if the object is being torn throughout its movements.
For example, Kenshiro's Hundred Cracked Fist
But, the speed lines here feel as it the object is radiating something.
Although, the speed lines of a person gunning down with finger bullets can't be compared to someone spiking a volley ball.
Let's take a look at the examples below from Haikyuu
Here we have two ways to convey motion, by blurring the object while in motion, or showing the afterimages of the object (because comics are sort of like photographs of moments) in motion.
3) Anatomy.
Don't worry about this, everybody has trouble with this.
But practice on anatomy.
4) Character Design
Here's a video from Gintama to teach you about character design.
Don't fret about too much, just something to keep note of.
5) Stage picture
Decide how much impact you want your characters to make in a panel and design the panel based on that impact you want to give. Use different shots or different angles to convey different moods or feel to add on to the writing.
For example, and tooting my own horn
Without looking at the text, you can see the shape of the characters and what they convey from their body posture. The man on the left is rather annoyed of the man on the left for some reason, and the man on the right is stoic, statuesque.
That's all the criticisms I'm going to give you as of now.
But you're doing okay. Just read a lot of other manga and draw influences from them to make a better manga. Study each panel and how they work. Feel free to experiment.
I realize that this may seem very meanspirited...