Resistors are pretty straight forward. They convert electrical energy to heat by resisting the flow of electrical current (kind of like friction resisting movement). Their resistance is measured in ohms, abbreviated to Ω.
For a nice little app to read resistor codes look here. I tend to cheat and just put a multimeter across the resistors though, but you do need to use the colour codes if they are already in a circuit.
I encourage all people starting out to simply buy a big multipack of resistors such as this one (rapid online, uk). This way you dont have to spend ages choosing additional ones for each project and wondering if you already have the ones you need.
For circuits where accurate resistance is required, such as my voltage regulator circuit, I recommend using 1% accuracy resistors such as the ones i linked above. Otherwise any will do the job.
be sure that the resistor will be able to handle the amount of current you are sending through it. The easiest way to do this is to use the following equation:
P = (V * V) / R
Where P is the resistor power, V is the voltage across the resistor and R is the resistor value
so a 10Ω resistor across 5volts would need be able to handle at least:
P = 5 * 5 / 10 = 2.5 watts
For more information than you ever need about resistors, visit the wikipedia page
1kΩ = 1000Ω
1mΩ = 1000kΩ