Remember - see your doctor before starting any weight loss plan.
Medifast, Jenny Craig and Nutrisystem are all plans where you buy pre-packaged meals that are portioned for weight loss.
Medifast has a nice online guide explaining their program here: http://www.medifastmedia.com/med/docs/quick_start_guide.pdf
With all these programs remember....you aren't going to buy their food for the rest of your life - but it's designed to teach you proper portion control. Having portioned meals can come in handy if you don't trust yourself in portion control.