To make a magnet repel a metal, one must first understand the properties of a magnet. A magnet has two poles, a north pole and a south pole. When magnets are placed near each other, opposite poles attract and like poles repel one another. When a metal enters a magnetic field, all of the electrons inside the metal "line up," causing a temporary magnetic alignment that is attracted to the magnet (the pole doesn't matter). That alignment dissipates once the magnetic field is removed and therefore, the only way for a metal to be repelled by a magnet is if it's first magnetized to the opposite pole.
Draw an "X" on one side of your magnet with a marker. This will simply let you keep straight which side of the magnet has one pole and which side has the other pole. The north and south designation of the poles isn't important, but you need to know which side is which.
Place your metal in close proximity to your magnet and make sure that it attracts. Some metal is non-magnetic and will not be attracted or repelled no matter what you do.
Rub the magnet over the metal in one direction many, many times. Don't vary and rub the magnet backward over your metal, as this will undo the magnetism. By rubbing a magnet over your metal continuously, you are aligning the electrons in the metal in a certain way, toward a certain polarity. The more that you do this, the longer the effect will last, taking more time for the electrons to return to their random or unpolarized state.
Take your magnet away from the metal and turn it around so the opposite end (the opposite pole) is now facing the metal that you just magnetized. Push the magnet toward the metal and, because the metal was magnetized by the opposite pole, the magnet should now repel the metal.
You can also go all out and create an electromagnet out of your metal that will create a magnet field that will either be attracted by or repelled by other magnets. To make an electromagnet out of your metal, wrap and coil wire around it completely and attach both ends of the wire to a battery. Once the circuit is closed, the metal (and the wires around it) will create a magnet and whatever magnet is nearby will be attracted or repelled by it. This is a much more complicated, but more permanent way to magnetize metal.