The reason you get different numbers is that the calorie calculators often use different formulas. Some use Harris-Benedict, some use Mifflin-St Jeor, etc. Also, all calorie calculators are mere estimates because we're all different. For instance, if I put in all my stats and use several different calorie calculators online, I'll get results that state that I can eat anywhere from 1800 - 2400 calories to maintain my weight. Through trial and error, I realize that I can eat 2200 calories per day to maintain my weight.
Here's a decent calorie calculator:
http://www.freedieting.com/tools/calorie_calculator.htm
My advice is to start with their number, and adjust your calories according to your progress (without putting too much stock into daily scale fluctuations if you happen to weigh daily).