If you want to move something in the direction it is facing, you need to convert the rotation to a vector using cos and sin. Something like this should move the bullet in the direction it is facing, assuming the bullet texture is pointing up (like the Godot logo):
global_position += Vector2(cos(rotation), sin(rotation)) * MOVE_SPEED
You might need to swap cos and sin though, I haven't tested the code and I have a tendency to get them mixed up.
Hopefully this helps!
(Side note: Welcome to the forums)
