Well, you're only 15. What is your height?
P90X is great if you are looking to lose weight, but the key to any weight loss is a proper diet. I believe the first three weeks is 50% protein, 30% carb, and 20% fat. I hate to say it, but you're going to have to eat a lot of:
Chicken
Tuna
Broccoli
Carrots
Salad
I'm 18, 5'10'', 160lbs, and on my cutting times of 50%, 30%, and 20%, I'm getting about 200g protein, 120g carbs, and 36g fat.
Again, diet is key here. You're going to hate certain things: cutting out cereal (yes, even whole grain...since you're only allowed 120g of carbs a day, you need to get the carbs from 1 or 2 fruits, protein shakes, protein bars, and A LOT OF VEGETABLES (but you can literally eat bags of vegetables for a handful of carbs)).
Don't cut out all protein bars. But I would only eat one since most bars contain 15-25g of carbs.
I personally like Pure Protein brand and Detour. Try to find a bar with less than 10g of sugar and 15g or more of protein, or something with a low glycemic index.
On a side note: if you don't have a lot of muscle under your "fat" (not sure how much you have...probably not much at 140 lbs), P90X isn't going to make you look amazing. The best thing would be to do a 6-8 week lifting round for bulk. Yes, you'll add fat, but you'll add loads of muscle as well. Then do P90X and expose those muscles.
I'm not 100% sure on the percentages for bulking. I've heard to do 1g per pound of body weight for protein, but I've also heard to take your caloric needs for the day (including activity level AND workout) and add 1000, then go 50% carbs, 30% protein, and 20% fat. Get a second opinion on this
