If you're looking to lose 100 pounds, how many calories you should eat to achieve that goal is going to depend on how fast you want to lose the weight.
Initially, you'll want to start off slow though and ease your way into the diet.
Start by eating 500 calories below your maintenance, that in and of itself will yield you at least 1 pound of loss/week.
You find your maintenance by multiplying your weight by 15. So for you, 270 x 15 = 4,050 calories/day.
So 500 calories less than this is 3,550/day.
That is still quite a lot of food to be eating each day, so you may want to eat even less. Every 500 calories you take off will net you 1 pound more of weight loss each week.
Ultimately though, you don't want to dip below 1,200 calories as a woman.
If you run into any plateaus, in your journey, up your cardio or your cardio intensity, eat fewer calories, or try cutting out your carb intake for 1-2 days by restricting your diet to mainly veggies and lean meats.
This should be good enough to get you started in the right direction, however if you want to make it all the way successfully, it would be a good idea to continue to educate yourself on how to properly setup an effective diet/exercise regimen.
You can find out more info on how to do that here.
