how do I make that when my character shoots his bullets they fall to the ground instead of being lightning fast shooting straight
Add drag to the bullet, lower the emit force, and maybe make the sprite bigger. A tiny sprite goes lightning fast even with a little force, due to how the engine determines mass.
Chances are that the bullet going straight is just an illusion, as its going so fast and disappears so quickly that the engine doesnt have time to actually drop it noticeably. Its probably actually dropping, but just insignificantly.