What kind of electric charges moves through metals to form an electric current?
In the absence of an electric field, the electrons in a conductor (metal) exhibit random motion i.e they move with different velocities in different directions. Hence, the net velocity of electrons is zero.
On application of an external electric field, the electrons start flowing towards the higher potential. The flow of charge (or electron) per unit time is defined as current.
Hence, it is the electrons that move through a conductor to form an electric current.
Keep in mind that the direction of the current is taken opposite to the direction of flow of electrons by convention.