In order to achieve a true vibrant unnatural hair color, you will need to bleach your hair prior to any coloring. Bleaching will remove the natural pigment and make hair more porous, allowing more dye to penetrate the hair shaft. Please read for an explanation on why this is recommended.
The decision to bleach your hair is an important one. Bleach is made of peroxides, ammonias, and lightening agents that remove the pigment melanin from the hair shaft by oxidative destruction. This process is permanent, and there is no way for your hair to magically go back to its natural color other than by letting it grow out. The good news, is if you want to go back to a natural color after bleaching your hair - you can always dye your hair a color similar to your natural shade as it grows in to make it less noticeable.
It is important to be very cautious when bleaching your hair, follow instructions, and don't leave the bleach in too long, Do not bleach more than once in a day - unless of course you WANT your hair to melt off and go bald (you don't). If it doesn't come out light enough the first time, wait at least a day before bleaching again. If you want to be prepared for the worst, buy a bottle of a darker "normal" hair color just in case it comes out awful and you are starting to have second thoughts!
...but at the end of the day bleaching your hair is all worth it, because when you see celebrities jumping on the platinum bandwagon, you're like...