the resistor is because LED's are generally designed to be used for around 1.5-4V depending on the LED it's self... buy them off of ebay... 90% of the companies that will sell you the LED's will give you free resistors, just specify that you want them for 12v...
that being said, different colour LED's use different size resistors, so make sure that your LED works with your resistor
Light Emitting Diodes (LEDs)
this will help you learn about LED's it also has a section about calculating the proper type of resistor for your application
lastly, i will point this out for you...
Super Bright LEDs - Instrument Cluster and Gauge Backlights
sometimes it's easier to buy something that someone has already made for your application... this is where i am going to be getting mine from, same with the ones for the back of my dash, but i gotta pull it all apart to count bulbs first