First of all you need to know how much money you earn, for some people this is easy you look at your Wθ form and there it is, but for seasonal workers, small business people, it gets a little more complicated than that. If you receive income from a lot of different sources for a realistic value of what you actually earn itemize them, put them down on paper. Include all income, now is not the time to be hide money from the IRS you are just fooling yourself, you need to have factual figures here of your true income otherwise you will actually think you're in good financial shape because you know you got this extra income somewhere, but that's not always true.
Include all variable expenses now, car maintenance, electricity, heating, these can vary throughout the year you need to calculate them and come up with an average monthly amount. None essential expenses like eating out coffee at Starbucks, weekend getaways, extravagant vacations, fancy clothing, motorcycles, ATV's, if you use them, write them down, realize these are wants, not needs. Now total these all up, fixed expenses, variable expenses, and non_essential expenses,there you now have your total expenses.