Resistors are used to change the amount of current flowing through a part of the circuit. This is often used as a means of protecting components which cannot handle large currents.
The current allowed to flow through a resistor can be calculated using: I = V/R (current = voltage/resistance).
There are many different resistors available, so they have a sequence of coloured stripes to show their resistance. See here for details on how to decipher the colour coding.
Resistors can be purchased from most electronics retailers.
For the purposes of these tutorials, any wattage rating is sufficient. If you later intend to use the resistor in higher-power circuits, you will need to ensure that the wattage is at least as large as the resistance multiplied by the current squared (P = I2R).