Hey there, and
To know how many calories you're eating, you write down or record everything you eat, and look up the calories. Some online free programs like Fitday or Sparkpeople will allow you to search for foods, select your portion, and give you the calories - they'll track what you've eaten in a day.
As for what you're burning in a day, that's really individual. There are no hard and fast rules. You don't typically want to eat less than 1200 calories (particularly since you're starting at a higher weight), and your individual loss might vary. I'd suggest starting at around 1500-1600 calories for a few weeks. If you're losing, stay that course. If you're not losing, lower the calories down a little bit more.
As for dealing with people in your household who don't need to lose weight, you have a few options. You can cook a lean protein and a vegetable for you both, and then provide a starch or other side for your son that you don't eat or eat a small portion of (for example, I know many people who make one pot of spaghetti sauce with ground turkey, and serve it over whole wheat pasta for the other family members and spaghetti squash for themselves). You can also cook meals that both of you can enjoy (because healthy food is good for EVERYONE, even people who don't need to lose weight...good food is good for your body regardless of weight), and serve your son a larger portion. I honestly prefer the second approach, because I believe in healthy eating at every size...even thin people need nutrients, even more if they are active, and I think there are plenty of things you can do with healthy foods so that everyone likes them.
We have some great information in our "Calorie Counters" forum on the basics of counting calories. You can also look at the 100 lb club, for those on their way to losing or who have lost 100 lbs, or the 40-somethings for people in that age range. You can post in one or many forums, so look around and jump in wherever you like.
Can't wait to get to know you better!