A 12 week challenge can be tough, particularly without an external support group. You will most likely endure high's and low's as your lifestyle goes topsy-turvey...but it is of so much benefit to have other's providing encouragement and advice throughout the challenge. Unfortunately not everyone will support you in your endeavours (as I experienced first hand myself), so I really feel that a support group is a vital factor in determining your success.
If you're unable to find family or friends who will undertake the challenge with you, hop on our discussion forum. During my challenge, I found a forum to be of excellent encouragement - particularly when you are discussing matters that everyone on the forum can relate to.
Also, consider a personal trainer. A 1on1 personal trainer is a great idea if you can find someone local who you feel comfortable with. Alternatively, online personal training can be just as effective. A personal trainer will provide you with direction, motivation, inspiration and knowledge - all key ingredients in getting through the 12 weeks successfully.
I hope this is of help and all the best of luck with your goals!