I have found this link
http://www.bmi-calculator.net/bmr-calculator/ to be the best BMR calculator (sorry I tried to just attach the link but it would not work)
After figuring out your BMR then go to the link for
Harris Benedict there you will see what you need to lose.
As far as determining your activity level, I had some of the same questions, and I lean toward the less active side, better to be safe than sorry.
These calculators are very accurate and what many professionals use.
Once you figure out what you need in a day for your activity level to maintain then deduct 500 calories per day if you want to lose 1 pound a week, or 1000 calories per day if you want to lose 2 pounds a week. Many people have the mind set that you need to be at 1000-1200 calories to lose weight, and that does not work. For most people that causes more harm than good--me being one of them. I need 1800 calories a day to lose 2 pounds a week. When I dip below 1500 calories I stop, dead in my tracks, and wont lose any more till I ease it back at least to 1650.
Hope that helps