Hello there Twin Mum and welcome to the site.
21lbs by September is definitely doable (how tall are you? and is that a weight you could reasonably expect to reach? did you use the Dukan calculator on the official website to see what it sets your "true weight" at, based on your age, number of children, diet history etc - it's far more accurate than the usual BMI one...).
While it's definitely possible to lose that amount of weight, you'll not have had time to consolidate it (as you'd need 8-10 weeks minimum for that after the loss) so if you've a holiday planned, you might need to be a little careful.
Foods I'd not be without? Definitely oatbran (and wheatbran for me); herbs and spices (I now use both far more than before, and rarely use salt!); err eggs... hum...; salmon; chicken; lean steak; prawns.
If you like that sort of food, you'll be fine... have you worked out your menu for day 1? You could post it (here or in the "what are you eating today?" thread...) and we could give you our comments?
The main thing in attack is not to be hungry... the first two to three days of this diet is where you might well feel hunger, so be sure to have plenty of Dukan friendly foods available (if you're at work) or in the fridge!
I always recommend emptying the fridge and cupboards of anything not ok too!
Good luck and don't hesitate if you have any other q's!
