The cars and trucks that we drive, the boxes we ship, the products we manufacture, the emissions from the food we eat, the air-conditioning we use in our buildings; all emit GHGs. Some people generate much more emissions than others, but it is impossible to live and not generate GHGs.
Carbon Footprint calculators are actually GHG footprint calculators and they will take you through the different emission categories, ask you questions about how you live and give you measures for each category. Category examples include:
Calculators are an exact science based on the Greenhouse Gas Protocol, but the results you get are of course as good as the information you put in.
It's important to be reasonably accurate, but don't let perfect get in the way of good. The results difference you are likely to get from diligently filling out every question (and uploading your entire household energy records for the past 10 years), versus taking the quick ball park option isn't massive.
If you are a detail person, go the whole question set, but the real point of knowing, at least roughly what your emissions are, is the decision to do something about it.
You can also go back and modify behaviours and see how much difference you can make. You are then armed with the information you need to start making changes in your life. Make sure you start with easy things that you hardly notice. You'll be surprised just how easy it is to make a big difference.
There are lots of excellent calculators around to help you work your footprint out and here are a few.
Footprint Calculator | Low Carbon Living | Carbon Neutral | WWF Footprint Calculator
Calculators vary slightly, but you can see how you rank against the rest of the population and green homes on a graph, save your result and check your overall performance. You will often also be offered offset options.