There are various ways to transfer the sufficient energy to free electrons in a metal and make them leave the surface.

(a) Thermionic emission: The metal surface is heated until the electrons gain sufficient energy to come out.

(b) Field emission: High electric field is applied across the metal to pull out the electrons

(c) Secondary emission: A high energy electron is made to hit the plate thereby transferring sufficient energy to other electrons to come out. 

(d) Photo electric emission is another way in which light with sufficient energy is shone on a plate and the electrons absorb this energy to eject out of the metal surface.