Simple induction heaters are made by creating a magnetic field to induce activity in a metallic substance, so you must create a coil that safely creates a magnetic field to induce electromagnetic activity. You need a circuit to handle the charge and an appropriate coil and capacitor with sufficient cooling.
The circuit requires resistors, diodes, transistors and inductors, and optionally you can use heat sinks to prevent overheating. An example of an induction heater provided by Research Media and Cybernetics uses two resistors, two diodes, two transistors, one capacitor and two inductors. The extra inductor is used to guard the rest of the circuit from the induction forces. It is possible to run cool water through your induction coil to keep it from overheating.
Induction heating is caused by the resistance of the metal multiplied to the total current flowing into it as a magnetic field. Metals that are more resistant heat up more easily. This effect is used in a great deal of appliances for adjusting the voltage. If you want to heat up a lower resistance metal, use a higher voltage. Higher resistance metals require less voltage. This effect is something that is usually avoided by designers, as it wastes energy.