Nice game, especially since it is your first! I have a few suggestions though: When you shoot, it looks like you ADD a given velocity to the player. This means that if you jump and shoot right after, you get crazy velocity, and if you fall and shoot, you get barely any velocity. Instead you should SET the player's velocity, that way they always get the same speed regardless of how and where they shoot. Second, the fullscreen doesn't scale assets. You can mess around with the Godot project settings: Display -> Window -> Stretch Mode = Canvas Items or Viewport (each have their own advantages and disadvantages). Finally, whenever I shoot, the bullet goes slightly above where I aimed. I'm not sure exactly why, and I would have to see your code to fix it myself, so I'm just letting you know about that.
Again, great job on your first game! Keep creating more!
Edit: If you have any questions on the coding tips above lmk