If I had a piece of metal and i wanted it to be negatively charged. How can I do that?
There are many ways to charge a piece of metal, but they tend to be variations on the principle used by a Van de Graaff generator. When you run two materials together you will usually transfer electrons from one to the other. Which way the electrons go depends on where the two surfaces are in the triboelectric series. Rubbing materials directly on the metal tends not to work as the charge simply flows off again. Typically you charge an intermediary object then touch that to the metal to charge it.
Alternatively find a radioactive beta emitter and point it at the metal then wait a long time.
Electrostatic Induction is good to use. It's the phenomenon of inducing electric charges without any direct contact with a charge. This principle is used in capacitors. Even, Rubbing materials produce static electricity. Insulators could be charged by rubbing. But, Metals are probably charged using insulators..! When you bring a charged plastic or glass rod (probably negatively charged) near a metal piece positive charges which experience attractive Coulombic force move towards the end of metal nearer to the rod, while negative charges move to the other end due to repulsive force. If you ground the metal piece, negative charges flow to ground while positive charges stick to the end (due to attractive force)... However after removing the ground, the positive charges are distributed throughout the metal piece.
Edit: After charging Insulators, charges could be transferred from Insulator to your metal piece by simply touching it..! (Thanks to @John)
Also, Van de Graaff Generator is based on both Electrostatic induction and Corona discharge (Action of points) to produce high voltage of the order of $10^7V$. But, it's been in use to accelerate ions for nuclear disintegration purposes instead of charging metal pieces..!
Previous posters have explained how to charge objects using various methods.
As metals are conductive, they cannot be permanently charged: the charge will leak away when it touches other things. The only way to keep it charged is to mount it on an insulator, but as no insulator is perfect, the charge will still leak away eventually. Even with a perfect insulator, charge will leak away through the air, so you also need to mount it in vacuum. You would need to keep the voltage low enough so the charge does not arc over to the surroundings.
There are materials, called electrets, that can keep an electric charge semi-permanently. The are special insulators that are charged by an electric field at high temperature. When the temperature drops, they keep their charge. While the charge does leak gradually, the best ones will keep a useful charge for many years. They are used in items like microphones.
A quick google search gave an article on making your own electret.