Ok, I really don't understand how many calories I am supposed to eat during the day.

I am 27, 250lbs and 6ft and I would like to get down to 210-215lbs and I am starting the lean version of P90X today. I would like to know how many calories I should go for because I don't understand the layout because it says I should be 3500 a day but then the plans I look at are for 1200 to 1500 a day. I just need to know where to start and any help would be great.

Thanks
Wade