Tail light mods are a popular topic right now, and there are plenty of people here ready to help.
Please take a few min, use the search feature above and read everyones tail-light threads, many of your questions will be answered.
specificly you'll find out why not to use 5mm leds, what mcd means and what viewing angle is appropriate and methods of having separate running and brake light brightness.
after you're caught up, come on back with details like:
is this a retrofit or a replacement, how much room do you have to work with, what brightness are you trying to achieve (that is, what brightness are the bulbs you're replacing)
don't buy any leds until you've got a fairly concrete plan!IMPORTANT INFORMATION: Your car / bike / rv / boat's 12VDC electrical system has some nasty surprises in store - read and understand this before messing with your brake lights:
Automotive EMC standard (why 12v is not 12v!) 
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